-m 








Class XL_i4A 



Book. 



hlA. 



CopjgMN? 



!^lt 



COEWRIGHT DEPOSIE 



WORKS OF PROF. F. P. SPALDING 

PUBLISHED BY 

JOHN WILEY & SONS 



A Text-book on Roads and Pavements. 

The aim of this work is to give a brief discus- 
sion, from an engineering standpoint, of the 
principles involved in highvray work, and to 
outline the more important systems of con- 
struction, with a view to forming a text which 
may serve as a basis for a systematic study of 
the subject. Fourth Edition Revised and partly 
Rewritten. 13mo, xii + 408 pages, 51 figures. 
Cloth, $2.00 net. 

Hydraulic Cement: Its Properties, Testing, and 
Use. 

This work embodies the results'of a careful 
study of the nature and properties of hydraulic 
cement, and the various methods which have 
been proposed, or are in use, for testing cement. 
Second Edition, Rewritten. ISruo, x + 300 
pages, 34 figures. Cloth, $2.00. 



A TEXT-BOOK 

ON 

ROADS AND PAVEMENTS 



BY ^J" 
FREDERICK P/ SPALDING 

PROFESSOR OF CIVIL ENGINEERING, UNIVERSITY OF MISSOURI, MBMBBR 
AMERICAN SOCIETY OF CIVIL ENGINEERS 



FOURTH EDITION, REVISED AND ENLARGED 
FIRST THOUSAND 



NEW YORK 

JOHN WILEY & SONS 

London: CHAPMAN & HALL, Limited 

1912 









Copyright, 1894, 1902, 1908, 1912, 

BY 

FREDERICK P. SPALDING 



SCIENTIFIC PRESS 

ROBERT DRUMMOND AND COMPANV 

BROOKLYN, N. Y. 






PREFACE. 



Successful practice in the construction of highways 
must depend upon correct reasoning from elementary 
principles in each instance rather than upon following 
definite rules or methods of construction. 

The aim of this book is to give a brief discussion, 
from an engineering standpoint, of the principles 
involved in highwaj^ work, and to outline the more 
important sj^stems of construction, with a view to 
forming a text which may serve as a basis for a syste- 
matic study of the subject. 

Details and statistics of particular examples have 
for the most part been excluded as undesirable in a 
book of this character. Such information is available 
in many forms for those having the necessary elemen- 
tary training and experience to enable them to properly 
use it. 

Considerable space has been given to the location 
and construction of country roads, as seemed proper 
in view of the present general public interest in the 
matter, and the probable development of this new field 
of activity in engineering work. The improvement of 
our common roads must come through transferring 
such work to the charge of those who make it a profes- 
sion, and not through teaching the public how roads 
should be constructed. 

F. P. S. 
Ithaca, N.Y., July, 1894. 



NOTE TO FOURTH EDITION. 



During the past few years, advances in the methods 
employed in the construction and maintenance of high- 
ways have been very rapid. Changes in the character 
of traffic, due to the introduction of automobiles, have 
presented new problems, while modifications in the 
standards of life, both in city and country, render the 
old methods no longer satisfactory to the pubHc. These 
changed conditions have caused more careful and scientific 
study of materials, and resulted in the use of more efficient 
methods, and in the development of new types of con- 
struction. 

In preparing the third edition, in 1908, it was found 
necessary to practically rewrite the entire book. In the 
present edition, new chapters are added on Bituminous 
Macadam and Concrete Pavements, while the chapters 
on Brick, Asphalt and Wood Pavements have been 
considerably modified. The size of the book has neces- 
sarily expanded considerably beyond its former limits. 

F. P. S. 
Columbia, Mo„ June, 191 2. 



CONTENTS, 



CHAPTER I 
Road Economics and Management. 

PACK 

Art. 1. Object of Roads i 

2, Resistance to Traction , 3 

3, Tractive Power of Horses ,....,...., 8 

4, Benefits Derived from Good Roads . 10 

5c Cost of Wagon Transportation ,....» 12 

6. Economic Value of Road Improvement , o . . . 16 

7- Sources of Revenue for Road Improvement ... o .,.„... , 19 

8. Systems of Road Management ,.....,..,.... 22 

CHAPTER IL 
Drainage of Streets and Roads. 

Art 9 Necessity for Drainage . o ., ... . 26 

10 Surface Drainage .....,..<, c ...... . 28 

II. Sub-drainage..... 29 

1 2 Tile Drains .,.„...,... = .,,... t,t, 

13. Stone Drains . , ..,..,....., o .... c . 36 

14. Culverts ,, o ........ o ,„ , 38 

15 Concrete Culverts , » 45 

CHAPTER TIL 
Location of Country RoadSc 

Art 16 Considerations Governing Location ........,.,.,. 49 

17. Length of Road. ... 52 

18 Rise and Fall , 55 

19. Rate of Grade. . = . 57 

20. Examination of Country .... .«,.,..-. 59 

21- Placing the Line . ......,,....,,.....-.,. 63 

22. Comparison of Routes ....<...«. 65 

23. Changing Existing Locations ,,..,.., 69 

vii 



CONTENTS. IX 

PAGE 

Art. 50. Penetration Method 197 

51 . Mixing Method 202 

52. Selection of Bituminous Materials 204 



CHAPTER VII. 

Foundations for Pavements. 

Art. 53. Preparation of Road-bed 209 

54. Trenches in Streets 210 

55. Purpose of Foundation 212 

56. Bases of Gravel and Broken Stone 214 

57. Concrete Bases 214 

58. Bituminous Foundations 217 

59. Miscellaneous Foundations 218 

60. Choice of Foundations 220 

CHAPTER VIII. 

Brick Pavements. 

Art. 61. Paving Brick 221 

62. Tests for Paving Brick 229 

63. Construction of Brick Pavements 249 

64. Filling of Joints 255 

65. Maintenance of Brick Pavements 261 



CHAPTER IX. 

Asphalt Pavements. 

Art. 66. Asphalt 263 

67. Asphaltic Cement 271 

68. Test for Asphaltic Cement 273 

69. Surface Mixtures 284 

70. Construction of Sheet Pavement 291 

71. Asphalt Blocks 296 

72. Maintenance of Asphalt Pavements 298 

73. Bitulithic Pavements 301 



viii CONTENTS. 

CHAPTER IV, 
Improvement and Maintenance of Country Roads, 

PAGB 

Art. 24. Nature of Improvements. 72 

25. Grade and Cross-section , o , 73 

26. Earthwork ,....., , . . . 79 

27. Earth Roads ..» ., 85 

28. Gravel Roads - ^ . . 93 

29. Oiled Roads = . , 96 

30. Sand-clay Roads « 107 

31. Miscellaneous Roads ,..,-, . iii 

32. Width of Tires .......00.115 



CHAPTER V. 
Broken-Stone Roads. 

Art. S3- Definition 117 

34. Macadam Roads 118 

35. Telford Foundations 120 

36. Rocks for Road Building 124 

S7. Methods of Testing Stone 137 

38. Road Metal 146 

39. Compacting the Road 151 

40. Thickness of Road Covering 153 

41. Maintenance of Broken-Stone Roads 156 

42. Dust Preventives 159 



CHAPTER VI. 
Bituminous IMacadam Roads. 

Art. 43. Types of Bituminous IMacadam 168 

44. Bituminous Materials 171 

45. Petroleums 174 

46. Solid Native Bitumens 177 

47. Tar Products 1 79 

48. Tests for Bituminous Materials 182 

49. Surface Treatment 192 



X CONTENTS. 

CHAPTER X. 

Wood-Block Pavements. 

PAGE 

Art. 74. Types of Wood-block Pavement 307 

75. Wood Blocks 311 

76. Treatment of Wood Blocks 315 

77. Tests for Wood Blocks 321 

78. Construction of Wood-block Pavements 322 

79. Maintenance of Wood-block Pavements 326 

CHAPTER XL 

Stone-Block Pavements. 

Art. 80. Stone for Pavements 328 

81. Cobblestone Pavements 330 

82. Belgian Blocks 331 

83. Granite and Sandstone Blocks 332 

84. Construction 334 

85. Stone Trackways 337 

CHAPTER XIT. 

Concrete Pavements, 

Art. 86. Concrete as Surface Material 339 

87. Portland Cement 340 

88. Portland Cement Mortar 343 

89. Portland Cement Concrete 345 

90. Mortar Surfaced Pavements 348 

91. Monolithic Concrete Pavements 354 

92. Grouted Concrete Pavements 356 

CHAPTER Xin. 

City Streets. 

Art, 93. Arrangement of City Streets 357 

94. Width and Cross-section 362 

95. Street Grades 367 

96. Street Intersections 370 



CONTENTS. XI 

PAGE 

Art. 97. Footways 372 

98. Curbs and Gutters 378 

99. Crossings 384 

100. Street-railway Track 385 

loi. Trees for Streets 398 

102. Selection of Pavements 399 

103. Sources of Revenue for Street Improvement 406 



ROADS AND PAVEMENTS. 



CHAPTER I. 

ROAD ECONOMICS AND MANAGEMENT. 
Art. I. Object of Roads. 

The primary object of a road or street is to provide 
a way for travel, and for the transportation of goods 
from one place to another. The facility with w^hich 
traffic may be conducted over any given road depends 
upon the resistance offered to the passing of vehicles 
by the surface or the grades of the road, as well as 
upon the freedom of movement allowed by the width 
and form of the roadw^ay. In order that a road may 
offer the least resistance to traffic, it should have as 
hard and smooth a surface as possible, while affording 
a good foothold to horses, and should be so located as 
to give the most direct route w^ith the least gradients. 

The expediency of any proposed road construction 
or improvement depends upon its desirability as affect- 
ing the comfort, convenience, and health of residents 
of the locality, and also upon its economic value, w^hich 
is largely determined by its cost and durability, as w^ell 
as upon the facility it gives for the conduct of traffic. 

The desirability of a road surface for any particular 
use depends both upon its fitness for the service 
required of it and upon its durability in use. 



2 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Upon a country road, the problem of improvement 
ordinarily consists simply in providing the hardest and 
most durable surface consistent with an economical 
expenditure of available funds, the object being to 
lighten the cost of transportation by reducing the 
resistance to traction, and to render travel easy and 
comfortable. 

Upon city streets, however, several other factors 
may be of importance in the design of highway 
improvements. 

The comfort both of those using the street and of 
the occupants of adjoining property will be largely 
affected by the freedom of the surface from noise and 
dust. 

The safety of the pavement in use, its effect upon 
the health of residents of the localit^^, and its economic 
value must in each case be considered. 

To adjust to the best advantage these various ele- 
ments, frequently quite discordant with each other, is 
a matter which can only be accomplished by the exer- 
cise of good judgment. Local conditions and necessi- 
ties must always be considered — such as the difficulties 
of drainage, the availability of various materials, the 
nature of the traffic to be carried, and the needs of the 
business or property interests of the neighborhood. 
Thus, for heavy hauling of a large city, the durability 
and resistance to wear of the pavement may be the 
paramount consideration; for an office district, quiet 
may be very important; for the lighter driving of a 
residence street, the elements of comfort and health- 
fulness may properly be considered as of greater force 
than the purely economic ones; while in all of the 
cases the necessary limitation of first cost will largely 
determine what may or may not be done. 



ROAD ECONOMICS AND MANAGEMENT. 3 

The problem of the highway engineer, in designing 
works of this character, involves the consideration of 
these various elements and their proper adjustment to 
give the best results. 

The kinds of road surface most commonly employed 
are as follows: For the streets of cities and towns, 
pavements of stone blocks, brick, asphalt, and wood; 
for suburban streets and important country roads, 
macadam and gravel surfaces; for ordinary country 
roads in general, surfaces of earth or gravel. 

Art. 2. Resistance to Traction. 

The resistance to traction of a vehicle on a road 
surface may be divided into three parts: axle friction, 
rolling resistance, and grade resistance. 

Axle friction varies with the nature of the bearing 
surfaces, and for vehicles of similar construction is 
directh^ proportional to the load. It is entirely inde- 
pendent of the nature of the road surface. 

Rolling resistance is of two kinds : that due to irregu- 
larities in the surface of the road, and that of a wheel 
to rolling upon a smooth surface, sometimes called 
rolling friction. 

The resistance due to an inequality in the road sur- 
face is the horizontal force necessary, at the axle, to 
raise the weight upon the wheel to the height of the 
obstacle to be passed. Thus (Fig. i), by the principle 

of the lever, P = W - • 
a 

For small inequalities, this resistance will be approxi- 
mately inversel}^ as the diameter of the wheel. The 
effect of small irregularities in the surface, however, 
is due more to the shocks and concussions produced by 



4 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

them than to the direct lifting action of the obstacle, 
and the resistance due to uneven surface is greater at 
high than at low velocities. 
X^^\ "^X Rolling friction is probably due 

/ \ for the most part to the compres- 

/ i I sibility of the surface of the road, 

Py ', \ I which permits the wheel to indent 

V ^ ; / it to some extent. The wheel is 

^ikC-\ ^^ thus always forcing a wave of the 

surface before it, or climbing an 

inclination caused by its weight 

upon the road surface. 

Size of Wheels. — The resistance to traction varies 
for wheels of differing diameters, being less for large 
than for small wheels. The experiments of M. Morin, 
in France, seemed to indicate that the resistance varies 
inversely as the diameter. Other experiments have 
indicated a less variation, approximately as the 
square root of the diameter, while Mr. D. K. Clark 
(Roads and Streets, by Law and Clark; London, 
1890) concludes, from a mathematical discussion based 
upon the assumption that the material of the surface 
is homogeneous and the pressure proportional to the 
depth of penetration, that the resistance to traction 
is inversely as the cube root of the diameter of the 
wheel. The experiments of Mr. Mairs (Bulletin, Uni- 
versity of Missouri Agricultural Experiment Station, 
1902) indicate that tractive resistance is somewhat 
less with large than with small wheels, being nearly 
inversely as the square root of the diameter, but as 
might be expected, differing somewhat for different 
road surfaces. 

For practical purposes it may be considered that, for 
wheels of ordinary sizes used on road vehicles, the 



ROAD ECONOMICS AND MANAGEMENT. 5 

rolling resistances are equal to the load multiplied 
by a coefficient which depends upon the nature 
and condition of the road surface, although these 
coefficients are somewhat affected by the sizes of the 
wheels. 

Width of Tire. — The effect upon tractive resistance 
of the width of tire upon the wagon wheels depends 
upon the character of the surface upon which the wheel 
is rolling. In a series of experiments at the University 
of Missouri Agricultural Experiment Station in 1897, 
it was found that wide tires considerably diminished 
tractive resistance upon broken stone and gravel roads, 
and upon earth roads in good condition, but upon 
muddy roads or when a hard road is covered with deep 
dust the resistance is greater for wide tires. The wide 
tire also has considerable advantage upon plowed 
land or sod, not cutting in so deeply. 

Speed of Travel. — Tractive resistances are somewhat 
greater at high than at low velocities. This difference 
is very slight on earth roads in good condition or on 
smooth pavements; but on rough pavements where con- 
cussions take place the resistance increases rapidly as 
the speed becomes greater. 

Road Surface. — Many experiments have been made 
for the purpose of determining the tractive force 
required for a given load upon various road surfaces. 
The results show somewhat wide variations, as would 
be expected when the many elements that may affect 
them are considered. The following table shows a 
few average results, which will give some idea of the 
relative resistances of various surfaces and of the 
advantage to be derived from a smooth and w^ell-kept 
road surface: 



6 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

TRACTIVE RESISTANCES ON VARIOUS SURFACES. 



Character of Road. 



Earth Roads — in fair condition . . . 
dry and hard ...... 

Macadam — very good. 

ordinary 

poor 

Granite block pavement — good . . . . 
ordinary 

Brick pavement 

Wood block pavement 

Asphalt pavement 



Resistance per 


Ton 




Pounds. 


ICO 


to 


175 


60 to 


125 


25 


to 


50 


40 


to 


100 


75 


to 


150 


25 


to 


50 


40 


to 


80 


20 


to 


50 


25 


to 


50 


20 


to 


70 



The resistance upon asphalt is greater at high than 
at low temperatures. 

Grade Resistances. — Tractive resistance due to grade 
is independent of the nature of the road surface or of 

the size of the wheels. 
It is equal to the load 
multiplied by the sine 
of the angle made by 
the grade with the hor- 
izontal. Thus in Fig. 2 
the tractive force P, 
due to the grade, is the 
force necessary to pre- 
vent the wheel from rolling down the slope under the 
action of the weight W, or it is the component of W 
parallel to the slope ac. 

he 




Fig. 2. 



P =w 



ac 



Grades are ordinarily expressed in terms of rise or 
fall in feet per hundred, or as percentage of horizontal 
distance. 



ROAD ECONOMICS AND MANAGEMENT. / 

For all ordinary cases of small inclinations ab is 
approximately equal to ac, and we may take 

be 
ab 
or the tractive force necessary to overcome any grade 
equals the load multiplied by the percentage of grade. 
The total tractive force necessary to haul a load up 
an inclined road equals the sum of the force necessary 
to haul the load upon the same surface when level and 
the force necessary to overcome the grade resistance. 
Thus, if we wish to find the tractive effort necessary to 
haul a load of 2 tons up a grade of 3 ft. in 100 over a 
good macadam road. Taking the resistance of the 
road surface when level at 60 lbs. per ton, we have for 
the total resistance 

i? = 2 X 60 + 4000 X y|-q- = 240 lbs. 

In going down the grade, the force due to grade 
becomes a propelling force, and the tractive effort 
required is the difference between the surface resist- 
ance and grade force. In case the grade force be the 
greater, the resulting tractive force becomes negative, 
or it will be necessary to apply the force as a resistance 
to prevent acceleration of the velocity in the descent. 

The angle for which the tractive force required for a 
given surface equals the grade resistance is called the 
Angle of Repose for that surface. In the case given 
above, 2 X 60 — 4000 X yw ^ ^' ^^ ^^^ angle of repose 
for a surface whose level resistance is 60 lbs. per ton is 
a 3 per cent grade. If a vehicle were left standing upon 
that inclination, it should remain standing with the 
forces just balanced. If it were started down the 
grade, it should continue to move at a uniform rate, 
without the application of any other force. 



8 a text-book on roads and pavements. 

Art. 3. Tractive Power of Horses. 

The loads that a horse can pull upon various road 
surfaces will not necessarily be proportional to the 
resistance offered by the surface to traction, as the 
tractive force that the horse can exert depends upon 
the foothold afforded by the surface. The ability of a 
horse to exert a tractive force depends upon the 
strength of the animal, upon his training for the par- 
ticular work, and whether he be accustomed to the 
surface upon which he is travelling. The work of dif- 
ferent animals is therefore subject to considerable varia- 
tions, and only very rough approximations are possible 
in giving average values of the work a horse ma3^ do 
under differing conditions. 

The tractive force that may be exerted by a horse, 
at moderate speeds, varie^ approximately inversely as 
the rate of speed; or, in other words, the power that 
a horse can exert through any considerable time is 
nearly constant for varying velocities. Thus it may 
be assumed, as an average value, that a horse working 
regularly ten hours per day can put forth a tractive 
effort of 80 pounds at a speed of 250 feet per minute 
on an ordinary level road surface. 

For the power of the horse we then have 

Power = force X velocity = 80 X 250 = 20000 foot- 
lbs, per minute. 

For any other rate of speed, as 200 feet per minute, 
we would have 20000 -^ 200 =100 pounds as the 
tractive force exerted by the horse. 

If the period of daily work be lessened, the power 
that may be developed will be increased, either by 
increasing the load or the velocity. 



ROAD ECONOMICS AND MANAGEMENT. 9 

The tractive force that a horse is able to exert 
decreases very rapidly as the rate of inclination 
increases. This is due both to the expenditure of 
power b}^ the horse in lifting his own weight up the 
grade, and to the less firm footing on the inclination. 
The effect of differences in the foothold afforded by 
various pavements is very marked in the loss of tractive 
power upon grades. 

In the table below are given the loads that an 
average horse may be expected to continuously haul 
up different inclinations, on various road surfaces, at 
slow speed. These figures, while of little value as an 
absolute measure of what may be done in any par- 
ticular case, are of use as a rough comparison of the 
relative tractive properties of different surfaces and 
grades. The effect of grades upon tractive effort w411 
also depend upon the condition in which the surface is 
maintained, and upon the weather. Snow and ice in 
winter, or the damp and muddy condition of some 
pavements in wet weather, have a ver3^ considerable 
effect to diminish tractive power. 

LOADS IN POUNDS THAT A HORSE CAN DRAW UPON VARIOUS 
SURFACES AND GRADES. 



Kinds of Surface. 



Earth road — good 

poor 

Broken-stone — good . . . . 

poor. . . . 
Stone Blocks — good . . . . 

poor 

Asphalt — clean and dry . 



Level. 



3000 
1300 
4000 
1600 
6000 
3000 
8000 



Rate of Grade. 



2400 
HOG 
2700 
1 100 
4500 
2300 
4000 



2 in 
100. 



2000 
goo 
2000 
800 
3300 
1700 
2500 



3 m 
100. 



1600 
700 
1600 
600 
2700 
1400 
1800 



4 in 
100. 



1400 
600 
1400 
500 
2200 
1 100 
1300 



5 m 
100. 



1200 

500 

1200 

450 
1700 

900 
1000 



10 in 
100. 



800 
400 
700 
250 
900 

450 
400 



15 in 
100. 



300 

150 
200 
100 
400 
200 



lO A TEXT-BOOK ON ROADS AND PAVEMENTS. 

In general, the tractive effort that a horse may exert 
is approximately proportional to the weight of the 
horse and the averages above given correspond to light 
animals; many horses are capable of exerting double 
the pull mentioned. 

A horse may frequently exert for a short time a 
tractive force about double that which he can exert 
continuously; hence, when short grades occur steeper 
than the general grades of the road, loads may often 
be taken over them much heavier than could be carried 
if the steeper grade prevailed upon the road. 

On ordinary country roads in dry weather the 
amount of load that can be hauled is usually deter- 
mined rather by the grades than by the nature of the 
surface. Unless the gradients are very light the 
amount of load that can be carried on a broken-stone 
surface does not differ greatly from what may be taken 
on a dry and hard earth road. In improving a road 
by substituting a hard surface for a surface of earth 
the gradients and location should therefore always be 
carefully studied, with a view to deriving the full 
practical benefit from the hard surface in the light 
traction that it may require with easy ruling gradients. 

Art. 4. Benefits Derived from Good Roads. 

The condition of the public highways is a matter of 
the most vital interest to any rural communit^^. Upon 
it depends largely the social life and enjoyment of the 
people living in the country as well as the ability to 
market the products of the farm to the best advantage. 

In nearly all parts of the country the roads are fairly 
good during a portion of the year; but there is also 
usually a period when they are very bad, in very 



ROAD ECONOMICS AND MANAGEMENT. I I 

man}^ localities becoming practicalh^ impassable. The 
improvement of the road surfaces and the use of 
systematic maintenance would make the roads better 
at all times, making it possible to haul larger loads 
over them and rendering them more pleasant to travel; 
but the most important object of road improvement is 
to eliminate the period when roads are not in condition 
to use and make it possible to drive upon thein and 
haul loads over them at all times. 

The benefits of good roads ma^^ be classified as social, 
educational, and financial. They promote social inter- 
course among the residents of a country district by 
making travel eas3^ and pleasant. Where the roads 
become impassable during a portion of the 3'ear, the 
residents are practicalh^ isolated at the period of 
greatest leisure and lose that intercourse with their 
neighbors which is a most important means of enjoy- 
ment and development. Attendance at church and 
public meetings is facilitated by good roads. There 
are man3^ localities where the condition of the roads 
practically closes the churches during a considerable 
portion of each j^ear, and in some instances they have 
been so deserted on this account as to be abandoned. 
The rural mail deliver3^ also depends for its efficienc\^ 
upon the good condition of the roads. 

The consolidation of rural schools and establish- 
ment of rural high schools, made possible by good roads, 
is an important advance in educational methods, and 
places rural communities more nearly on an equalit^^ 
with the cities in educational advantages offered to 
children. 

Roads which can be traveled all the year admit of 
marketing the products of the farm at any time which 
may be most advantageous, enabling the farmer to take 



12 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

advantage of favorable market conditions and prices, 
or to transport his products at a convenient season, 
when he can do the work without interference with 
other duties of men and teams. 

The condition of the highways has also considerable 
effect upon the business of the towns into which they 
may lead; where they are uniformly good throughout 
the year mercantile business will be better distributed 
between different seasons, and a larger volume of 
business will be transacted. The same effect is pro- 
duced upon railway transportation. Congestion in rail- 
way business and scarcity of cars is frequently the 
result of the hurried marketing of crops to take advan- 
tage of a good condition of the v/agon roads, and a 
much better distribution of business might be obtained 
through an improved condition of the highwaj^s. 
The area tributary to a town or a railway may also 
frequently be considerably extended by road improve- 
ment. 

The greatest benefits derived from good roads are in 
the increased comfort, convenience, and pleasure of the 
people living near them, and in the social and edu- 
cational advantages which they make possible and 
which add greatly to the attractiveness and happiness 
of rural life. 

Art. 5. Cost of Wagon Transportation. 

The effect of bad roads upon the cost of wagon 
transportation has been the subject of much discussion 
and many estimates have been made which have 
arrived at widely different conclusions. Many of 
these discussions have failed to take account of all the 
factors entering into the problem and have arrived at 



ROAD ECONOMICS AND MANAGEMENT, 1 3 

wildly extravagant results. Efforts have been made 
b}^ the Road Inquiry Office of the United States 
Department of Agriculture to collect statistics con- 
cerning the volume and cost of hauling farm produce to 
market for the whole United States. These statistics 
include estimates of the average length of haul and the 
cost per ton-mile, with a view to basing upon them some 
conclusion as to the saving to the country in general 
which would result from the improvement of the roads 
so that larger loads may be carried, and less labor be 
required for moving the crops. Statistics of this kind 
are very difficult to gather, being altogether dependent 
upon the judgment of the man who collects them in 
each locality, and representing only very approximately 
the average conditions. They are of value as giving 
information concerning the traffic to which the roads 
are subject in various localities and the need for road 
improvement, but they do not contain data upon which 
anj^ reasonable estimate can be based of the actual 
saving which might be eft'ected b^^ road improvement. 
Such general estimates are not of 2Lny particular value 
other than that of showing something of the size of the 
problem when applied to the whole countr3\ 

Many palpably erroneous and exaggerated estimates 
of the saving in cost of transportation by road improve- 
ment have been published and have often seriously 
injured the cause of good roads. Thej^ aim to show 
the large saving which may be effected by the farmer 
through reducing the cost of moving his crops to 
market, but their fallacies are evident to the farmer 
who reads them and applies them to his own conditions, 
and in manj^ instances lead him to doubt the good 
faith of the whole movement for good roads. These 
estimates commonly treat the subject as though the 



14 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

whole of the crops were hauled to market in full loads 
by teams kept by the farmers for that purpose alone, 
and which could be dispensed with if the roads were so 
improved as to require a less number of loads, and con- 
sequently less teams to transport the crops, which is 
clearly not the case. 

The effect of the condition of the highways upon 
the cost of wagon transportation depends upon the 
character of the traffic. Where this consists of the 
transportation of some product which is hauled in full 
loads, with teams which are employed for this purpose 
only, the cost of transportation is readily ascertained, 
and the saving due to any improvement which increases 
the load carried b3^ each team may be found by esti- 
mating the cost of the decreased number of teams 
required. If an earth road in poor condition be re- 
placed by a good macadam surface, the load which 
can be taken over the road may easily be doubled if 
upon light gradients, while where a traffic of this 
character must be taken over an earth road in bad or 
muddy condition, the construction of an improved 
road surface may result in loads four or five times as 
heavy as before. In this case the number of teams is 
inversely proportional to the maximum load which maj^ 
be hauled and the cost is proportional to the number 
of teams. This condition soemtimes, though rarel^^ 
occurs upon wagon roads, the traffic usually being of a 
mixed character, with varying percentages carried in 
full loads, and with teams kept for other purposes and 
only incidentally used for transportation upon the 
roads. 

For the purpose of estimating the cost of trans- 
portation upon ordinary country roads it is necessary 
to separate the traffic into classes and determine what 



ROAD ECONOMICS AND MANAGEMENT. 1 5 

portion of it is carried in full loads. This is ahvaj's a 
matter of difficulty where the traffic is varied and 
can onh^ be done in a very roughly approximate 
manner. The light portion of the traffic will, of course, 
be benefited b}' improved roads, but the saving in 
cost of conducting the traffic, while existent, is 
usualh^ comparativeh^ small and practicalh^ indeter- 
minate. It consists in saving time of men and teams 
through greater speed of travel, and in less wear 
upon teams and vehicles. When such traffic must be 
conducted over muddy and bad roads these items 
maj^ be of considerable importance, although the\" can 
not be evaluated, but commonlj^ they are of slight 
importance. 

The lieav}^ portion of the traffic is more directh" 
affected by the character of the roads over which it 
passes. This traffic is carried in full loads, which are 
limited in amount b}^ the condition and gradients of the 
roads. In some localities this constitutes the main 
portion of the traffic; in others it is a comparatively 
small part of the whole. No generalization concern- 
ing the value of road improvement as reducing the cost 
of transportation can therefore be made with any 
approach to accuracy; but in a particular instance where 
data is obtainable concerning the heavy traffic, it is 
possible to roughh^ estimate the saving in labor of 
transportation through road improvement. If we 
can determine the cost of using teams for this purpose, 
an approximate estimate may also be made of the 
saving in cost of transportation through such improve- 
ment. The cost of using teams for highwa}^ trans- 
portation is often difficult to obtain on account of the 
fact that such teams are commonly kept for other 
purposes and onh^ incidentalh^ used for road work. 



1 6 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

and in some instances it is possible that the trans- 
portation is done when there is no other work which 
could be done by the team. In general, however, it is 
fair to assume that the cost of the work is proportional 
to its amount, and that if the teams were not employed 
in transportation on the highway, they would be other- 
wise usefully engaged. In estimating the cost of work 
of teams, the actual cost to the farmer of keeping the 
team should be used and not the rental value of teams 
in the vicinity. 

A careful examination of the local conditions sur- 
rounding the traffic is essential to any reasonable 
estimate of saving to be effected in cost of transpor- 
tation upon highways. Such estimates are not of 
much value at best as giving actual amount of savings, 
but studies of this kind may be of value in giving a 
better conception of the economics of the good roads 
problem. 

Art. 6. Economic Value of Road 
Improvement. 

The value of a road improvement to a community 
and the amount of money that may reasonably and 
profitably be expended in the construction and main- 
tenance of common roads is a subject the discussion of 
which leads different persons to widely different con- 
clusions, depending upon the point of view and the data 
assumed. Any improvement, either in position or 
surface, that has the effect of increasing the loads that 
may be taken over a road by a given power lessens the 
number of loads necessary to carry the traffic, and 
effects a saving in time and labor of men and teams, 
which may reasonably be considered to have the same 



ROAD ECONOMICS AND MANAGEMENT. 1/ 

money value as the time used in the work. This has 
been discussed in Art. 5 and is the most direct and 
obvious financial gain which may result from road 
improvement. 

Saving in cost of transportation is not, however, 
the most important advantage to be gained by road 
improvement, and if it were the only one, in many 
instances, the expenditure of money necessary to 
secure better roads could not be justified as econom- 
ically profitable. 

It is in wet and muddy weather that improved 
surfaces have their chief advantage over earth roads, 
and the main object of introducing hard and imperme- 
able surfaces is to eliminate the period when ordinary 
earth roads are apt to be muddy and practically 
useless for the purposes of transportation, and to 
substitute a road that may be used at any season. 
Systematic drainage has a similar object. To a farm- 
ing community the economic advantage of a road 
uniformly good at all seasons is greater than might 
appear at first glance. It may in many instances 
amount practically to a saving equal to nearly the 
entire cost of hauling by permitting the work to be 
done at times when other work is impossible, thus 
making men and teams available for other duty in 
good weather. The ability to use a road at any 
season is also of advantage in the independence of 
weather that will make it possible to take advantage 
of the condition of the markets in the disposal of 
produce or purchase of supplies. These advantages 
may be of greater or less importance according to the 
character of the traffic carried by the road. In general, 
while they are indeterminate and can not be expressed 
in money value, they are evidently of more economic 



1 8 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

importance than the saving effected in costs of trans- 
portation. 

The nature of the country roads affects the towns to 
which the country is tributary as well as the country 
itself. They directly affect trade in seasons of bad 
weather, both in regulating the demand for supplies 
for country consumption and in controlling the supply 
of produce which is available for market; indirectly 
also the prosperity of a rural district means that of its 
trade center. The improvement of country roads is, 
therefore, of direct economic value to towns into which 
they may lead, but this, like most of the other advan- 
tages of good roads, is dependent upon data which can 
not be accurately estimated. 

All of these points must be considered in any at- 
tempt to arrive at any proper conception of the advan- 
tages of a proposed improvement. In any particular 
case the local interests will determine the relative 
importance of the various elements, and a careful 
analysis of the trade that does pass over the road and 
that would pass over it under different conditions will 
enable a judgment to be formed as to the value of 
improvements. The attempt to base an estimate of 
the economic value of a proposed road improvement 
upon the prospect of direct financial return is, however, 
apt to be misleading and to leave out of account the 
most important benefits of such improvements. The 
social and educational benefits mentioned in Art. 4 
are of highest importance and have also an economic 
value in their effect upon the desirability of a locality 
as a place of residence. The economic importance 
of good roads is shown in their effect upon land values, 
which are largely affected by them. 

The money spent in road improvement is to be 



ROAD ECONOMICS AND MANAGEMENT. 1 9 

considered as an investment, which will return annual 
interest to the community in reduced costs of trans- 
portation, greater freedom of traffic and travel, and in 
the increased comfort and happiness of the people. 

Art. 7. Sources of Revenue for Road 
Construction. 

Various methods have been employed for securing 
the funds necessary for the construction and improve- 
ment of countr}^ roads. Many of the earlier roads in 
this countr}'- were toll roads built bj^ private capital 
and kept up exclusiveh^ by charges paid b3^ travelers. 
Toll roads are objectionable because they impose a 
tax upon all the traffic of the road, and also because 
the cost of management is usuallj^ large, thus restricting 
traffic. They are conducted for the purpose of deriving 
a profit from their operation. They are gradually 
disappearing and should be dispensed with except 
under exceptional circumstances. 

District Roads. The most common inethod of rais- 
ing funds for road purposes is by property and poll 
tax in small districts. By this method a small poll 
tax is assessed against each voter of the district, 
pa^^able either in money or labor, and a certain property 
tax which is levied uniforml3^ upon property in the 
district, and often also paj^able in labor or cash at the 
option of the property owner. In some instances 
the property tax must be paid in monej^, and in others 
a portion of it must be paid in money. In some poor 
and sparsely settled localities the poll tax is the 
principle source of revenue, and road work consists 
mainh^ in working out the road tax by the residents 
of the district. 



20 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

As a general thing, the results of the collection of 
road tax in labor are not good. Under the labor 
system men do not work on the roads a sufficient 
length of time to become expert enough to do good 
work; many of them feel no particular interest in the 
work and are only concerned in getting in the required 
time. Under this system it is not usually possible 
to have the road work done when it is needed, as the 
convenience of the laborers must be considered. In 
many places therefore efforts are being made to require 
the payment of property tax in money. This is 
highly desirable wherever the money will be expended 
under proper management, but in numerous instances 
no improvement has followed a change to the cash 
system because of a lack of intelligent control of the 
work. 

County Road Tax. In some states there is a general 
property tax for road and bridge purposes. This 
tax is, usually, at the disposal of the county commis- 
sioners, or county court, which uses it for the con- 
struction and maintenance of county bridges and also 
for such road work as may be of special importance and 
of interest to the county in general. Frequently 
such funds are used to encourage the construction of 
improved roads by paying a part of the cost where 
owners of property specially benefited, or road dis- 
tricts in which the road may lie, are willing to pa3'^ 
their share of the cost. The maintenance of the 
ordinary country roads usually depends upon the 
district tax, but a county road fund judicious^ 
administered may do much to improve the more im- 
portant highways and thus secure good roads leading 
from various parts of the county to the market 
towns. 



ROAD ECONOMICS AND MANAGEMENT. 21 

Special Assessments. The laws of some of the 
states provide for levying special assessments against 
propertj^ benefited bj^ the improvement of comitry 
roads. These laws are usually permissive, allowing 
either* the county court or majority of property owners 
concerned, by instituting proper proceedings in court 
or before commissioners, to have the improvement 
made and assessed upon propertj'- within certain 
distances from the road. In some instances the 
whole cost is paid in this way; in others a part is 
appropriated b3^ the county, or state, or both, and a 
part is raised b3' special assessment. 

State Appropriations. Several of the states appro- 
priate state funds, which may be used in assisting in 
the construction of improved roads throughout the 
state. The amount of assistance given varies in 
different states; in New Jersey 2iZi P^^ cent of the 
cost is paid by the state, 56! per cent by the county, 
and 10 per cent by the abutting property; in Massachu- 
setts the state pays 75 per cent, and the county 25 per 
cent; in New York the state pays 50 per cent, the 
county 35 per cent, and the town 15 per cent. 

These various laws all have for their object the 
distribution of cost upon all interests benefited by 
the work. There has been much discussion of this 
subject and many opinions expressed concerning the 
justice of various methods of distributing the cost. 
The improvement of main roads by local districts 
without outside assistance does not seem equitable, 
and the use of general coimty tax for this purpose is 
intended to place a share of the cost upon the towns 
and other interests not reached in the local taxes. 
It also secures a centralized and usually better control 
of road work. All of the sources of revenue are 



22 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

benefited by road improvement and may reasonably 
be expected to contribute to its cost, but the exact 
evaluation of relative benefits is not possible, and the 
amount each must pay should be determined by con- 
sidering what is feasible and in what way the greatest 
improvement may be effected. 

Art. 8. Systems of Road Management. 

Several different systems for managing the work of 
constructing and repairing country roads have been 
proposed or are in use in various places. These 
systems differ in the placing of the control of the roads 
and in the methods adopted for providing funds. 

The control of the roads under the various sj^stems 
may be vested in the national government, in the vari- 
ous State governments, in county or parish organiza- 
tions or in townships or districts. In regard to the 
location of control and responsibility, it may be 
remarked that there are two points to be kept in view. 

1st. In order that the work may be economically 
conducted, the section of country included under one 
control should be sufficient to warrant the permanent 
employment of a man, or corps of men, whose business 
it shall be to continually look after the roads, study 
their needs, and systematically conduct their improve- 
ment. It should admit of the ownership and use of 
labor-saving machinery for the economical execution 
of the work, but should not be large enough to require 
an elaborate and complicated organization. 

2d. The control of road work should be so arranged 
that, as nearly as possible, all of the interests directl3^ 
affected by the condition of any road shall have a 
voice in its management and contribute to its support. 



ROAD FXONOMICS AND MANAGEMENT. 23 

Common roads are essentially local in their character 
and are not usually employed as lines of continuous 
transportation over any considerable distance. They 
are not, therefore, of state or national importance as 
lines of communication, although as factors in the 
general welfare of the people the3^ must, of course, like 
all other such factors, be of general interest and concern 
to both state and nation. 

The nation, and in most cases in this countr^^ the 
state, is too large a unit to assume direct control of 
road work. In general, the interests over so large an 
area are so varied, and the requirements so different, 
as to prevent a harmonious and successful organization 
of such work with a probability of economical adminis- 
tration. In some cases, however, such control might 
be wise and proper, and the recognition of the impor- 
tance of road improvement to the general welfare of 
the state, through the payment b3" the state of a 
portion of the cost of permanent improvements, has 
in some instances proved a powerful stimulus to local 
action. 

The control of road management b}^ towns and small 
districts is nearly always inefficient because the organi- 
zation is too small to support a proper management 
or provide the necessary appliances for economic work. 
Under this system the man in charge of the roads is 
usually engaged in other work; he is not a road engineer, 
and can, and is expected to, give but little attention to 
the road work. This system of control is also usualh^ 
unfair, except in case of roads intended for the accom- 
modation of the local district only. For instance, a 
road passing through a town may be a thoroughfare 
for the towns upon each side. The principal traffic 
may be this through-trade to points beyond the limits 



24 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

of the town in which the road is situated. The cost of 
keeping up this road is largely due to outside traffic, 
and the intermediate town should not be required to 
bear all the expense of maintenance. On the other 
hand, the interests of the towns whose trade passes 
over the road are largely affected by its nature, and 
the people of thes.e towns should be permitted a voice 
in determining the character of the road. Most of the 
more important roads of every vicinity pass thus 
through several towns, and the system of improvement 
by small districts works injustice both ways — upon 
those who are obliged to keep a road for the use of 
others and upon those who are obliged to use a road 
they cannot cause to be kept in proper condition. 

County management seems more successful in this 
country than any other, as a county, or two counties 
combined if necessary, is usually strong enough to 
secure intelligent management and homogeneous 
enough to have common interests. 

The proper management of the common roads in 
any community requires both experience and intelli- 
gence. A man to be efficient in such work must be 
able to make or modify location where necessary, 
judge of the value of various materials for purposes of 
construction, determine the necessity for and means to 
be adopted for drainage, and possess the executive 
ability to manage men and control scattered work. 
The work in each locality is a problem by itself, to be 
solved by careful study of the requirements of the 
community, taking into account the local natural 
conditions and available materials and means. 

Several of the states have State Highwa3^ Commis- 
sions, or State Highwa}^ Engineers, for the purpose of 
promoting the improvement of the country roads. 



ROAD ECONOMICS AND MANAGEMENT. 25 

These commissions are usually mainly advisory in 
character. They investigate and report upon the 
condition and needs of the roads, advise the local 
authorities concerning the best methods of construction, 
furnishing plans and specifications if desired, and 
control the expenditure of state funds applied to road 
purposes. These commissions have accomplished much 
in the way of improving existing conditions in several 
states, and have done much toward creating sentiment 
favorable to the expenditure of funds for such work. 
In Missouri the new road law provides a combined 
county and district management. It creates the office 
of. County Road Engineer to be appointed by the 
count}^ court. The County Engineer has direct charge 
of the expenditure of the general county road and bridge 
fund, and all district road supervisors in the county 
are required to report to him and to conduct their work 
under his general direction. The purpose is to provide 
a competent central authority in each county, w^ithout 
changing the existing division into road districts. 
There is also a State Highway Engineer whose duties 
are to advise with the local authorities, and to dis- 
tribute and control the expenditure of state appro- 
priations in aid of road improvement. 



CHAPTER 11. 

DRAINAGE OF ROADS AND STREETS. 
Art. 9. Necessity for Drainage. 

The road-bed, usually formed of the natural earth 
over which the road or pavement is to be constructed, 
must always carry the loads which come upon the 
road surface. Where an artificial road surface or 
pavement is employed, the earth road-bed is protected 
from the wear of the traffic, and the wheel loads com- 
ing upon the surface are distributed over a greater 
area of the road-bed than if the loads come directly 
upon the earth itself; but the loads are transferred 
through the pavement to the road-bed, and not sus- 
tained by the pavement as a rigid structure. 

The ability of earth to sustain a load depends in a 
large measure upon the amount of moisture contained 
by it. Most earths form a good firm foundation so 
long as they are kept dry, but when wet the3^ lose 
their sustaining power, becoming soft and incoherent. 
When softened by moisture the soil may be easily 
displaced by the settling of the foundation of the 
road, or forced upward into any interstices that may 
exist in its superstructure. 

In cold climates the drainage of a road is also impor- 
tant because of the danger of injury from freezing. 
Frost has no disturbing effect upon dry material, and 
hence is an element of danger only in a road that 
retains water. 

26 



DRAINAGE OF ROADS AND STREETS. 2/ 

In order, therefore, that the loads may be uniformly 
sustained, and the surface of the road kept firm and 
even, it is evidently of first importance that the road- 
bed be maintained in a dry condition. The improve- 
ment and maintenance of a road are therefore largelj^ 
questions of drainage, the object being to prevent water 
from reaching the road and to provide means for 
immediateh^ removing such as does reach it before the 
soil becomes saturated and softened. 

Surface drainage is alwaj^s necessary if the body of 
the road is to be kept in a dry condition, and is accom- 
plished b^" having the surface of such form that water 
falling upon it will quickly run into the gutters. 

The necessity for underdrainage in an\^ case depends 
upon local conditions, the nature of the soil and the 
tendenc^^ of the site to dampness. Underdrains are for 
the purpose of lowering the level of ground water in wet 
weather and preventing water from underground 
sources reaching the road bed and softening it. A 
careful examination of local conditions is necessary 
in cLuy case to determine the advisability of constructing 
underdrains. Where the soil upon which the road is 
constructed is so placed that the ground water is at 
any time likeh^ to stand close to the surface and become 
soft immediateh^ under the road-bed, underdrainage 
is necessarj^ to good results in the maintenance of the 
road. In anj^ case in which the level of ground water 
stands within about 3 feet of the surface, the road will 
be benefited by sub-surface drainage, although it uicly 
not be altogether necessary to the maintenance of the 
road. Underdrainage is of little use for the removal 
of water from depressions in the surface of the road. 



28 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Art. 10. Surface Drainage. 

The drainage of the surface of a road is provided for 
by making the section higher in the middle than at the 
sides, with ditches or gutters at the edges of the road 
along which the water is conducted tintil it may be dis- 
posed of through some side channel. 

The slope necessary from the middle to the sides of 
the road to insure good drainage depends upon the 
nature of the road surface, being less as the road surface 
is more smooth and less permeable to water. For 
ordinary earth roads it varies from about I in I o to 
I in 20; for macadam or gravel roads, from I in 15 
to I in 30; and for brick or asphalt pavements, from I 
in 40 to I in 60. 

The drainage of the surface of a country road is 
mainly a matter of maintenance, and involves keeping 
the surface of the road in a smooth condition and 
properly crowned. It is more fully discussed in Arts. 
25 and 27. 

On country roads the disposal of surface water is not 
usually a matter of difficulty, as it can be carried along 
the road and run into the first convenient cross channel. 
In deep cuts or on steep grades, however, it may some- 
times be economical to lay a pipe under the gutter 
into which surface water may be turned at frequent 
intervals. 

In all cases it is important that the water which 
falls upon the surface should be gotten rid of as soon 
as possible, for so long as it remains upon the road it 
is an element of danger, both from its tendency to 
wash the surface, and from its liabilitj^ to penetrate 
into the road and thus cause disintegration or settle- 
ment. 



DRAINAGE OF ROADS AND STREETS. 29 

Art. II. SUBDRAINAGE. 

The drainage of the subsoil under a road is intended to 
lower the level of ground water in wet weather and 
prevent water from sub-surface sources reaching the 
road-bed. 

The necessity for subdrainage, and the method to 
be employed in any case, depends upon whether the 
soil over which the road is being constructed is natu- 
rally wet or dr3^, and whether the road-bed is so situ- 
ated and formed as to give it natural drainage. 

The material of which a road-bed is composed is 
important because it determines to a large extent 
whether artificial drainage is necessary, and also what 
method should be adopted for securing drainage. 

Soils differ in their power to resist the percolation 
of water through them, in the rapidity and extent of 
their absorption of water with which they come in con- 
tact, in the extent to which moisture renders them soft 
and unstable, and in their power of retaining moisture, 

A light soil of a sandy nature usually presents little 
difficulty in the matter of drainage, as, while it is easily 
penetrated by water, it is not retentive of moisture, 
which passes freely through it without saturating it 
unless prevented from escaping. 

If the natural drainage, therefore, have a fall away 
from a road-bed formed of such material, it will usually 
need no artificial drainage, and where subdrains are nec- 
essary the^^ may be relied upon to draw the water from 
the soil to a considerble distance each side of the drain. 

A nearly pure sand is more firm and stable, under 
loads, when quite damp than if dry, although a fine 
sand saturated by water which is unable to escape 
may become unstable and treacherous. 



30 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Clays usually offer considerable resistance to the 
passing of water through them, and are very retentive 
of moisture. As a rule, however, a clay soil does not 
absorb water readily, and requires that water be held 
for some time in contact with it in order that it may 
become saturated, although when saturated it is the 
most unstable of soils. A clay that when dry will 
stand in a vertical wall and support a heavy weight, 
when wet may lose all coherence and become a fluid 
mass. When water comes in contact with a bed of 
such clay, the outside becomes saturated and semi- 
fluid before the moisture penetrates into it sufficiently 
to even moisten it a few inches from the surface. 

A clay soil is, therefore, always difficult to drain by 
removing the water after it has soaked in, or by per- 
mitting it to pass through the road-bed to the subdrains 
beneath^ Drainage, in such cases, may often be so 
arranged as to prevent water from standing against 
the road and thus prevent it from becoming saturated. 
As the clay is comparatively non-absorptive, the water 
which may come upon its surface, if allowed to escape 
at once, will not penetrate into it, and hence will not 
cause softening. 

A heavy silt formation is sometimes met with which 
is even more difficult to drain than a true clay. It is 
nearly as retentive of moisture as a clay, strongly 
resisting the passage of water through it, but at the 
same time absorbs water quite freely when in contact 
with it. 

Between the extremes mentioned above there are 
a great number of varieties of soil which possess to a 
greater or less extent the characteristics of either or 
both, and gradually merge the one into the other. In 
applying a system of drainage in any case, careful 



DRAINAGE OF ROADS AND STREETS. 3 1 

attention should ahva^'s be given to the characteristics 
of the soil, as determining very largely the treatment 
to be used. 

Where artificial subdrainage is necessary the drains 
should be located, in so far as possible, with a view to 
cutting off the supply of water before it reaches the 
road-bed. To accomplish this to the best advantage 
the local conditions must be observed, the sources of 
this supply determined, and the nature of the under- 
flow, if any exist, considered. In most instances on 
roads over soil commonlj^ met upon country roads a 
single line of tile under one side of the road will lower 
the ground water sufficiently to prevent it reaching 
the road-bed. 

Frequently, as in many cases of a road along a side 
slope, there is a well-defined flow of sub -surface water 
from one side to the other, and in such case the water 
may perhaps be intercepted by a single longitudinal 
drain on the side of the roadway from which the 
water comes. An example of this is shown in Fig. 3. 




Fig. 3. 

When the subsoil is of stiff and retentive material 
which does not drain readilj^, an underdrain on one side 
may not draw the water from under the whole width 
of the roadway. In this case it is advisable to use a 
drain on each side to cut off the water before it reaches 
the roadwa^^ This ma\^ be necessary with a clay soil 
when the line of ground water is high. 

Sometimes a single drain is laid under the middle of 



32 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

the road, as shown in Fig. 4. This is, in general, an 
undesirable practice; the middle of the roadway is 
not a convenient place for the drain, and necessitates 
digging a trench under the roadway which is likely 
to give considerable trouble in the early maintenance 
of the road surface. In some instances however a 
spring of water may come up under the roadway, as in 
a clay spout, and when this occurs it is desirable to lay 
a pipe to take the water from the source of supply 
rather than to drain it through the soil to the side 
drains. 

The most satisfactory and cheapest method of under- 




FlG. 



drainage is commonly by the use of porous drain tile, 
as used for farm drainage. Where stone is plentiful 
and handy to the road, a stone drain may be cheap and 
equally effective with the tile. These types of drains 
are described in succeeding articles. 

Many road builders utilize the side ditches, intended 
for surface drainage, for underdrainage also by making 
them deep and narrow. This is not usually an eco- 
nomical practice. A tile drain and shallow gutters 
will not be more expensive to construct than the deep 
ditches, while they are much easier and cheaper to 
maintain. In some instances tiles are laid under the 
surface ditches and the trenches filled with stones, or 
gravel, as shown in Fig. 5, thus permitting the surface 
drainage to seep into the tile. This gives very effective 



DRAINAGE OF ROADS AND STREETS. 33 

drainage, if the tile be of sufficient capacity, but is 
expensive to construct. 

In considering the advisabihty of underdrainage and 
the method of accomplishing it, the fact should be 
kept in mind that the purpose of underdrainage is to 
remove ground water, and that efficient drainage of 




Fig. s. 

the road surface can only be accomplished by main- 
taining the surface in smooth condition and of proper 
form. 

Art. 12. Tile Drains. 

Tile drains for road drainage are constructed in the 
same manner as for land drainage. Ordinary porous 
tiles are used as in farm drainage, sizes from 4 to 8 or 
TO inches in diameter being commonly employed for 
this purpose. They are usually in lengths of slightly 
more than 12 inches, the excess of length being sufficient 
to allow for probable breakage, so that estimates may 
be made on the basis of one tile to each foot of length. 
The tiles should be truly cylindrical with the ends cut 
off square, and be smooth inside. They are laid in a 
trench 3 or 4 feet below the surface of the ground, with 
their ends in contact. They should be carefully placed 
so that the ends fit closely, and the bottom of the 
trench should be cut to about the width of the tile, so 
that they cannot move sideways when the trench is 



34 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

filled; or better still, a groove may be scooped out in 
the bottom of the trench to fit the tile. 

Grade of Tile. The velocity of water through a tile 
depends upon the slope of the tile. Considerable water 
may be carried by a tile laid to an almost level grade. 
Such grades are, however, rarely necessary in road 
drainage and are to be avoided whenever possible. It 
is not desirable, except under unusual conditions, to 
use a grade less than about 2 inches per 1 00 feet. This 
gives a velocity which may be reasonably expected to 
keep the tile clear, when properly laid, except for small 
tiles, although a greater slope is to be preferred when 
obtainable. 

Care should be used in laying tile to place it accu- 
rately to the grade line, particularly when the slope is 
light. Irregularities are apt to produce depressions in 
which deposit of silt may take place. 

Size of Tile. A tile for road drainage should not be 
less than 4 inches in diameter. While a smaller tile 
may often be large enough to carry the water, the 
danger of clogging is much greater and the effect of 
inequalities in grade are increased for such tile. The 
size of tile required depends upon the quantitj^ of water 
to be carried and the slope of the tile. For agricultural 
drainage it is common to assume that the tile must 
remove from ^ inch to I inch of water per day over the 
area which it drains. The rules commonly followed 
probably give an excessive run-off in most instances, 
and recent observations indicate that I inch would be 
ample in most instances of ordinary drainage. This 
method may be applied in road work where the area 
from which the water is drawn can be determined. 
The area to be included depends upon the character 
of the soil and the way the ground lies. On level 



DRAINAGE OF ROADS .AND STREETS. 35 

ground the drain may be assumed to receive water 
from a certain distance on each side depending upon 
the porosity of the soil. 

For road drainage the size of tile used should be 
such as to provide liberal capacity. Comparatively 
small sizes will usually be required and the differ- 
ences in cost are small. An area of 25 to 50 feet on 
each side may be considered as contributing water 
to the tile. In ordinary soil the effect of the tile will 
reach much farther than this, but the percolation is 
so slow that the water will reach the tile very 
gradualh\ 

This method may serve as a guide in selecting the 
size of tile required; but is not capable of accurate 
computation and is only of value as an aid to judg- 
ment. Good practice in such work must rest mainly 
upon the judgment derived from experience. If the 
tile be supposed to collect w^ater from about 25 feet 
on each side, it would drain about an acre for each 
870 feet of length, or about 6 acres per mile. Assum- 
ing one-half inch in 24 hours, over the drainage area, 
as the amount to be provided for, one acre will yield 
1 81 5 cubic feet per day, or ij cubic feet per minute; 
and one mile of length, 50 feet wide, will j^ield about 
7^ cubic feet per minute. 

The water carrjdng capacity of tile drains has not 
been accuratel3^ determined but it probabl}^ does not 
greatly differ from that for vitrified pipe sewers, and 
the use of the formulas usually applied to sewers will 
be sufficiently accurate for practical purposes. The 
common formula for the flow of water, v = C ^RS, may 
for our purpose be transposed into the form V = k ^DS, 
in w^hich V is the velocity in cubic feet per second, 
D is the diameter of the pipe in inches, 5 is the slope. 



36 



A TEXT-BOOK ON ROADS AND PAVEMENTS. 



and fe is a coefficient varying from about 9 for 4-inch 
pipe to 12 for 12-inch pipe. 

The following table for the capacity of tile drains is 
based upon this formula. It is computed by the use 
of Kutter's formula, using a coefficient of roughness 
of .013, which corresponds to the flow in pipe sewers. 



CAPACITY OF 


TILE 


DRAINS IN 


CUBIC 


FEET PER 


MINUTE 


Slope per loo 
Feet. 


Sizes of Pipe. 


In. 


Feet. 


4 In. 


6 In. 


8 In. 


10 In. 


12 In. 


2 . 


.67 


4- 


12 . 


27. 


49-5 


81. 


4 






33 


5 


•5 


16.S 


38 




70 




114. 


6 






50 


6 


■5 


21 . 


46 


5 


86 


5 


143. 


9 






75 


8 




25-5 


57 


5 


106 


5 


176. 


12 




I 




9 


5 


29-5 


66 




122 


5 


204. 


24 




2 




13 


.S 


41-5 


92 


5 


173 




288. 


3(> 




3 




16 


5 


51- 


114 




212 




353- 


48 




4 




19 




59- 


132. 




245 




408. 


60 




5 




21 




66. 


148 




275 




456. 



Tiles laid upon very flat slopes sometimes may carry 
a quantity of water greater than the capacity due to 
the slope; this is caused by the level of ground water 
standing above the tile, thus causing the water to 
flow in the tile under a head greater than that due 
to its slope. Where gravel or other porous material 
is available such tile will be benefited by a porous 
filling immediately over the tile. This also assists in 
keeping the tile clear of sediment. 



Art. 13. Stone Drains. 

In localities where stone suitable for such purposes 
exists along a roadway it is common and often econ- 
omical to use stone drains for purposes of under- 
drainage. 



DRAINAGE OF ROADS AND STREETS. 3/ 

Blind Drains. For short lengths, where it is only 
necessar}^ to provide a permeable path for a small 
quantit}^ of water to escape, blind drains maj^ be used. 
They consist of ditches cut into the soil and filled at 
bottom with fragments of stone, the trench then 
filled with earth. Care should be taken that the top 
of the stone is protected, so that the earth may not 
be washed into the stone and stop the drain; a little 
small-sized stone or gravel on top, or a light layer of 
brush or sod, to hold the earth until it has compacted, 
is useful. Such drains have frequentlj^ proven quite 
efficient when used where the requirements are not too 
great. 

Box Drains. Wliere suitable stone is plenty and 
cheap, a box drain may be built. This consists of a 





Fig. 6. Fig. 7. 

rectangular box formed of flat stones at the bottom of 
the trench, which is then filled with earth. This box 
may be ver3^ roughly built, and it is desirable when 
stone or gravel is plentiful to fill immediately over 
the drain w^ith such material, to protect it against the 
entrance of earth and assist in leading the water into 
it. Figure 6 shows a section of such a drain as con- 



38 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

structed to intercept a seepage of water in stiff reten- 
tive material. In ordinary soil there would be no 
advantage in filling the trench so full of stone. The 
construction shown in Fig. 7 is also sometimes used, 
and is cheap and reasonably efficient. 

The size of opening in a stone drain must be consid- 
erably larger than that in a tile to carry the same 
quantity of water, the construction usually being very 
rough, and the resistance to flow greater. Drains of 
this type are used in many localities where materials 
are available for building them, although their use is 
growing less, due to the fact that porous tile costs so 
little and tile drains are so easily and cheaply con- 
structed. 

Art. 14. Culverts. 

Culverts are commonly required in road construction 
for carrying under the road the small streams which 
may be crossed by the road, or sometimes for carry- 
ing the water collected in the gutters or ditches on 
the upper side of the road to the lower side. 

The waterway provided by a culvert must, for 
safety, be sufficiently large to pass the maximum flow 
of water that is likely to occur, while for economj^ it 
must be made as small as may be without danger. 

The maximum flow of a stream depends upon a 
number of local conditions, most of which are ver3^ 
difficult of accurate determination. These are: the 
maximum rate of rainfall, the area drained b}^ the 
stream and its position, the character of the surface 
drained, and the nature of the channel. 

The maximum rate of rainfall varies in different 
localities, and differs in the same locality from year to 
year. It is commonly taken at about an inch an hour. 



DRAINAGE OF ROADS AND STREETS. 39 

This is sometimes exceeded for a very short time and 
over a small area, but is usually a safe value for a 
watershed of any considerable area. 

The approximate area of the watershed drained by 
a stream is readih^ found, and its form is also impor- 
tant as determining the distance the water must flow in 
reaching the culvert under consideration, and to some 
extent regulating the rate at which the water falling 
upon the area will reach the culvert. 

The maximum flow of a stream is also affected by 
the physical characteristics of the watershed. The 
permeability of the surface largelj^ determines what 
portion of the rainfall shall reach the stream; while the 
slope of the surface, its evenness, and its vegetation 
have an effect upon the quickness and rate with which 
the rainfall is received b^^ the stream. 

The determination of the maximum flow to be ex- 
pected in any case from an examination of the localit^^ 
is therefore possible onh^ as a very rough approxima- 
tion. A number of formulae have been proposed for 
such estimation, the use of which for the case of an 
ordinary culvert simply amounts to estimating the 
quantity of water which would fall on the w^atershed in 
the heaviest probable rain, and judging as well as pos- 
sible from local conditions how much of it may arrive 
at one time at the culvert. In some cases where a 
more accurate determination is desirable it may be 
advisable to measure the flow of the stream at high 
water, and form an idea from such measurement as to 
what ma^^ be expected at a maximum stage. 

The amount of water that will pass a culvert in a 
given time depends upon the form of the section, the 
smoothness of its interior surface, its slope, and the 
head under which the water is forced through. A 



40 A TEXT^BOOK ON ROADS AND PAVEMENTS. 

well-constructed culvert may be considered in comput- 
ing its capacity as a pipe flowing full. Other culverts 
or bridges must be treated as open channels. 

Prof. Talbot gives (Selected Papers C. E. Club, Univ. 
of Illinois, 1887-8) a formula for the rough determina- 
tion of area required for waterway, derived from ex- 
perience : 
Area waterway in feet = C V (drainage area in acres) .^ 

C is a coefficient depending upon local conditions. 
For rolling agricultural country subject to floods at 
time of melting snow, and with length of valley 3 or 4 
times the width, C = J. When the valley is longer, 
decrease C. If not affected by snow and with greater 
lengths, C may be taken at J, ^, or even less.. For 
steep side slopes C should be increased. 

For most cases in practice the size of waterway 
required may be determined from the knowledge which 
usually exists in the vicinity regarding the character of 
a stream, from the sizes of other openings upon the 
same stream, or from comparison with other streams 
of like character and extent in the same localityo 
Where data of this kind do not exist, careful exami- 
nation of water-marks on rocks, the presence of drift, 
etc., may be made to determine the height to which 
water has previously risen. The shape of the valley 
and the slope of the surface is of more importance 
than the area of country drained. The use of a for- 
mula like Talbot's assists the arrangement of the 
factors which enter into the determination, and is 
only intended as an aid to judgment in selecting the 
size of opening required. 

The discharging capacity of a culvert will depend 
upon the slope of the water surface passing through 
the culvert. Increasing the slope of the bed of the 



DRAINAGE OF ROADS AND STREETS. 4 1 

culvert will increase its carrying capacity, provided 
the water can flow freely away below the culvert. If 
a culvert be so constructed as to permit the water 
to dam up^ above it, causing the culvert to flow full and 
under pressure, the effect is the same as increasing the 
slope and increases the capacity of the culvert. The 
velocitj^ through a culvert is approximately^ propor- 
tional to the square root of the head of water, the 
head being the difference of elevation of the water 
surface at the entrance to the culvert and that w^here 
the water leaves the culvert. 

There are three types of culverts in common use for 
road purposes: stone box culverts, pipe culverts, and 
concrete culverts. In some locaHties wooden box cul- 
verts are also used for this purpose; these are very un- 
economical on account of their perishable nature and 
their use should be abandoned. 

Stone Culverts. Culverts of stone may be either 
arch culverts or box culverts. Box culverts are usually 
formed of two side walls and a cover. The side walls 
consist usually of rubble stonework laid up drj^ or in 
mortar, as the case may be. Where the stream to be 
carried is of small importance, and the capacity of the 
culvert not greatly taxed, dry walls maj^ give satis- 
factory results, but when the culvert is likely to flow 
full at certain times it should be laid up in hydraulic 
cement mortar, and in any case the greater stability 
given by the mortar would be well worth the small 
additional cost. Fig. 8 shows a section of the ordi- 
nary form of box culvert. The use of head walls and 
paving the waterway for a short distance is necessary 
for these, as for pipe culverts. 

Where suitable stone is available, box culverts are 
easily constructed and economical. They are com- 



42 



A TEXT-BOOK ON ROADS AND PAVEMENTS. 



monly used for openings 2 to 4 feet in width and 
2 to 5 feet in height. The width that may be used 
depends upon the available cover stones. Where the 
allowable width is not sufficient to give the needed 
area of waterway, a double culvert may sometimes be 
used to advantage. This consists of two openings 
with a middle wall to support the covers. 

The culvert's opening should always be large enough 
to admit of a man passing through it for the purpose 



mw/ 






'W/////W// 




W/////A 




y////M 


v////// 


% 


im 


1 


1 



Fig. 8. 

of cleaning it — at least 18 by 24 inches. The side walls 
should extend downward below the bottom of the 
culvert sufficiently to obtain a good foundation, and 
the thickness required for the side walls usuall}^ varies 
from one-half to three-fourths the height, depending 
upon the pressure likety to come against them. 

In many cases for small work the side walls, instead 
of extending downward, rest upon the paving which is 
extended under them. This gives a somewhat less 
expensive construction, and is often satisfactory^ on 
good ground. 

The cover stones may be from J to J the span in 
thickness, and should be long enough to have a bear- 



DRAINAGE OF ROADS AND STREETS. 43 

ing upon each side wall of at least one-half the thick- 
ness of the wall. 

Pipe Culverts. Pipe culverts ina3^ be constructed 
either of salt-glazed vitrified sewer pipe, or of iron 
water pipe. For culverts of sizes up to about 30 inches 
diameter, vitrified pipe is often the most economical 
material to use provided it is placed on good founda- 
tion and sufficiently covered not to be subject to shock 
from the traffic. The iron pipe possesses greater 
strength, and is preferable where a firm foundation is 
not easih^ obtained, or where a sufficient covering can 
not be had for the vitrified pipe, as it is not so easily 
broken by a slight settlement or b3^ shocks. It is 
somewhat expensive and not economical for ordinary 
use. 

In laying pipe culverts, the\^ should be placed on a 
solid bed, and the earth be well tamped about them. 
It is desirable to have the bottom of the trench exca- 
vated to fit the lower part of the pipe, depressions 
being formed for the sockets. It is necessary in every 
case that the pipe be firmly and uniformly supported 
from below, in order that the culvert may not be 
broken by settlement, which is especially likely to 
occur in new work. 

The joints in the pipe should be made water-tight, 
especially where the culvert is likely to flow full or 
under pressure, as any water escaping through the 
joints will tend to cause a wash beneath the pipe and 
undermine the culvert. Joints are commonly filled 
with clay, but where strength is needed the use of 
hydraulic cement mortar is preferable. The cost of 
filling joints is small and adds much to the security 
of the culvert. 

Care should be taken that the culvert have sufficient 



44 



A TEXT-BOOK ON ROADS AND PAVEMENTS. 



slope and be so placed that water may not stand in 
it, in order to prevent injury from freezing. When this 
is not feasible, iron pipe should be used. The top of 
the culvert pipe should be at least 1 8 inches below the 
road surface to avoid crushing, and for the larger sizes 
of pipe (24 to 36 inches), at least two feet. 

The ends of pipe culverts should be set in masonry 
walls to give protection against the washing of the 
face of the embankment, hold the ends firmly in place, 
and prevent the entrance of water into the earth on 
the outside of the pipe. 

These walls to give efficient protection must be of 
substantial construction, going down, to a solid founda- 
tion below the bed of the stream. They may be built 
of rubble masonry, and should be laid up in hydraulic 
cement mortar. Such construction is represented in 
Fig. 9. The wall must extend far enough on the side 




Fig. 9. 



to sustain the earth of the embankment from the 
waterway, or wing walls may be used extending up 
stream for this purpose. The waterway should be 
paved above the culvert far enough to prevent scour- 
ing at the base of the wall. 



DRAINAGE OF ROADS AND STREETS. 45 

For quite small streams the walls may sometimes 
be omitted if the face of the embankment about the 
entrance to the pipe and the waterway for some dis- 
tance above and below be riprapped. \%ere it is 
necessar}^ to economize in the cost of construction, 
this method is preferable to the use of very light end 
walls. 

On streams too large for a single pipe it is often 
economical to lay two or three pipes side by side, 
rather than to construct an arch or the open way of a 
bridge. In laying large pipes it is usually advisable 
to place a broken-stone or concrete foundation under 
the pipes throughout their lengths to insure uniform 
support. 

Art. 15. Concrete Culverts. 

Where the waterway required is too large to permit 
the use of vitrified pipe, concrete culverts are, in most 
instances, the most economical to use, and in many 
locations they may be placed more cheaph^ than the 
larger sizes of pipe culverts. Concrete, made of good 
materials, and properly mixed and placed, is a very 
durable material and will last indefinitely. A well 
designed concrete culvert should therefore require very 
little in the way of maintenance. 

These culverts are built either with arched or flat 
tops. For small spans, the rectangular box form is 
usually the most economical. The arched culvert for 
small spans is usually built of solid concrete without 
reinforcement, and is heavier than the box form, unless 
the culvert be very small. For longer spans the rein- 
forced arch is desirable. 

Fig. 10 shows the section of a concrete culvert in 
which the sides and bottom, as well as the top, are 



46 



A TEXT-BOOK ON ROADS AND PAVEMENTS 



y'l-AhVi / r-isz: 



reinforced with steel rods, for the purpose of taking 
the tension due to the tendency to bend under the loads 
which come upon it. Concrete is a good material for 
resistance to compression, but offers slight resistance 
to tension; the introduction of steel rods to take the 
tensions, therefore, make it possible to construct the 
walls and top of the culvert much lighter than they 
could otherwise be built. 

Structures of this kind must be carefully designed 
and constructed in order to secure good results. The 
steel should consist of small rods well bedded in the 
concrete. They should be placed with the center of 

the rods about two 
inches from the inner 
surface of the concrete. 
The area of steel re- 
quired in the top of 
such a culvert is usu- 
ally about one per 
cent of the area of the 
concrete above it. 

Concrete culverts, 
like pipe culverts, must 
be protected by a covering of earth from the shocks 
of the traffic. This covering should be 1 8 inches in 
thickness, and in no case should be less than 12 inches. 
The ends of the culvert must be protected by walls, 
which should extend at least two feet below the bottom 
of the culvert. 

The thickness of the top and sides must depend upon 
the loads which may come upon the culvert and upon 
the character of the concrete. Thej^ should visualh^ 
be designed to safely carry a heavy road roller. The 
concrete should be made of the best grade of Portland 



Fig. 10 



DRAINAGE OF ROADS AND STREETS. 



47 



cement mixed with good qualit}^ sand and gravel, or 
broken stone, so as to produce a very dense, homo- 
geneous concrete, the proportions for the top being 
about I part cement, 2 parts sand, 4 parts broken 
stone; for the sides and- bottom, i part cement, 2j 
parts sand, 5 parts broken stone or I part cement, 3 
parts sand, 6 parts broken stone. The following tables 
give approximate dimensions for culverts suitable for 
countrv roads under these conditions: 



TOP OF CULVERT. 



Span in feet ... 

Thickness of concrete, — inches. . 

Size of steel bars, — inches square 

Distance apart of bars, c. to c. — 

inches 

BOTTOM OF CULVERT. 

Span in feet 

Thickness of concrete, — inches. . 

Size of steel bars, — -inches square 

Distance apart of bars, c. to c. — 

inches 

SIDES OF CULVERT. 

Height of opening, — feet 

Thickness of concrete, — inches. 
Size of steel bars, — inches square 
Distance apart of bars, c. to c. — 
inches 











3 


4 


5 


6 


8 


9 


10 


II 


1 


I 


1 


1 


6 


8 


7 


6 


3 

5 


4 
6 


7 


6 
8 
f 


iS 


J2 


10 


16 


2 
6 


3 
6 


4 
7 


5 
8 


i 


1 


5 

8 


i 


12 


10 


12 


10 



13 



It may sometimes be desirable to leave out the con- 
crete bottom, and extend the side walls deeper, as with 
a stone box culvert. Where this is done, the side walls 
should extend at least 18 inches below the bottom of 
the culvert, and should widen at the bottom into a 
footing which will give a firm foundation to the 
structure. 



48 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

For small culverts on country roads the side walls 
may be of plain concrete, with a thickness of about 
one-third of the height. For the smaller sizes this 
may in many instances be cheaper than the reinforced 
sides. For openings not more than 1 8 inches to 2 feet 
square a semicircular arched opening, without rein- 
forcement, with a thickness of arch and side wall of 
about one-third the diameter, may be cheaper than the 
rectangular form of opening. 

In work of this kind, great care must be taken to 
secure good materials; the broken stone should be of 
good hard material, not too uniform in size, varying 
from about f-inch to I J or 1-2^ inches; sand should 
preferably be coarse and not uniform in size, it should 
be clean, hard sand; cement should meet the specifi- 
cations of the American Society for Testing Materials 
for Portland cement. The mixing and placing of the 
concrete must be carefully done so as to secure a 
thorough and uniform mixture of the ingredients, and 
a dense, compact mass of concrete in the culvert. 



CHAPTER III. 

LOCATION OF COUNTRY ROADS. 
Art. 1 6. Considerations Governing Location. 

The determination of a line for a proposed road 
involves the examination of the country through which 
the road is to pass with reference to its topographical 
features, the nature and extent of the traffic that it 
may develop, and the local interests that may be 
affected by the position of the road. 

The simplest form that this problem can take is that 
in which two points, as two towns, are to be connected 
by a road for the purpose of providing for a traffic 
between them, the nature and amount of which is 
approximately known. In this case it is only necessary 
to examine the topography of the intervening country 
and select the line over which, taking into account 
the costs of construction and maintenance, the given 
traffic may be most economically carried. 

In most cases in practice, however, the problem 
does not have this simple character, and in fact location 
can seldom be determined by considerations of economy 
alone. The position of the line will be modified by 
local needs, such as the necessity of providing for the 
traffic of villages or farms intermediate between the 
ends of the road, which maj^ often cause deviations 
from what would be the best line if the interests of 
the terminal points alone were considered. 

Questions of the desirability of various lines for the 
comfort and convenience of travel, and the pleasure to 

49 



50 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

be derived from the use of the road, dependent upon 
aesthetic considerations, may also frequently operate 
to change the line from what would seem proper from 
a strictly economic point of view. 

In thickly settled communities, as in most parts of the 
United States, the roads are in the main already located, 
the necessity for the location of new ones does not often 
arise, and when it does occur is usually mainly deter- 
mined by the local needs and requirements of traffic. 

The economic considerations involved in the location 
of roads are of tw^o kinds : those relating to the accom- 
modation of traffic, and those relating to its economic 
conduct. The first deals with the necessity of the road 
to the community, the second with the cost of operat- 
ing it. The first involves the general question of the 
advisability of any road, and how it can be placed to 
give the greatest freedom to the movement of travel. 
The question is as to the value of the road to the gen- 
eral community and its location to secure the greatest 
good for the least outlay, without taking into account 
the details of location which may affect the cost of 
transportation. The value of the road as developing 
trade in a town or bringing a farm nearer to market 
would enter into consideration. The accommodation 
of traffic requires that a road be located with a view to 
the convenience of its use by the largest portion of the 
traffic, as well as with a view of developing traffic. 

The position of a road that will best accommodate 
traffic is that in which, other things being equal, the 
mass of traffic need be moved the least distance in 
reaching its destination; or, in other words, that for 
which if each ton of freight be multiplied by the dis- 
tance through which it must be moved the summation 
of the resulting products will be a minimum. If th^re 



LOCATION OF COUNTRY ROADS. 5 1 

be differences in the nature of the routes over which 
the road may be constructed, the3^ maj^ be considered 
as equivalent to changes in the relative effective lengths 
of line for purposes of comparison. 

The ordinarj^ problem of location deals mainly with 
considerations of the second class. It consists for the 
most part in the relocation of portions of old roads, 
of making such changes in position when improving a 
road as may tend to reduce the cost of conducting 
traffic over it and render it more convenient and 
pleasant for the use of travel, or of determining the 
details of alignment and grade upon a new road which 
is approximately fixed in position bj" the purpose of 
its construction. 

The most economical location is that for which the 
sum of the annual costs of transportation, the annual 
costs for maintenance, and the interest on the cost of 
construction is a minimum. 

The cost of conducting transportation is affected by 
the rate of grade of the road, the amount of rise and 
fall in it, and the length of the road. The rate of 
grade is important, because it limits the loads that can 
be hauled over the road, or determines the number of 
loads that must be made to transport a given weight 
of freight, as well as fixes a limit to the speed of travel. 
The amount of rise and fall affects the expenditure of 
power required to haul a load over the road. The 
length of the road has an effect upon the amount of 
work necessarj^ to haul a load o'7er it, the time required 
for a trip, and the cost of maintaining the road surface; 
each of which, other conditions being the same, is 
direct^ proportional to the length. 

The cost of construction depends upon the accuracy 
with which the line of the road is fitted to the surface 



g2 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

of the ground, as determining the amount of earth- 
work and cost of bridges and culverts; upon the 
character of the ground over which the road is to be 
built, which affects the cost of executing the work and 
determines the necessity for and expense of drainage; 
and upon the cost of land for right of way. All of these 
items must be considered in any comparison of the 
cost of constructing on various routes. Special care 
should be taken in selecting a line to avoid bad ground, 
such as swamps, upon which construction may be diffi- 
cult and expensive. The availability near the line of 
the road of materials needed for surfacing may also 
become a matter of importance in the cost of construc- 
tion, and have an influence in determining location. 

The relative importance of the various elements 
affecting the choice of a line depends upon the nature 
and amount of the traffic to be provided for and upon 
the character of the road surface to be used. Where 
the traffic is heavy, the importance of reducing the 
cost of moving it by lessening grades and distance will 
be greater than where the traffic is light, and the cost 
of construction may be correspondingly increased for 
that purpose. If a smooth surface be employed, upon 
which traction is light, the value of reducing grades 
will be greater and the value of reducing distance less 
than with a surface of poorer tractive qualities. 

Art. 17. Length of Road. 

Changes in the length of a road affect all portions of 
the traffic in the same manner, and the expenditure of 
power and loss or gain in time occasioned by them are 
in general directly proportional to their amounts. 

The value of any considerable saving in length ma3^ 
usually be considered as equal to the same percentage 



LOCATION OF COUNTRY ROADS. 53 

of the whole cost of conducting the traffic that the 
saving in distance is of the whole length. If, therefore, 
a rough estimate may be made of the annual traffic to 
be expected, upon a given line of road and of the cost 
of carr^^ing the traffic, this cost divided by the length 
in miles through which the traffic is moved will give 
the annual interest upon the sum that ma^^ reasonably 
be expended in shortening the road one mile, or upon 
the value of a saving of a mile in distance; or dividing 
by the number of feet of distance will give the value of 
saving one foot. 

It is to be noted, however, that the cost of the work 
of transportation is not necessarily proportional to the 
amount of work done, and consequently this method 
would not be strictty accurate even were the data as 
to traffic and costs readily obtainable. An estimate of 
this character at best amounts to onlj^ a rough guess, 
but it Yn3.y often be of use as an aid to the judgment 
in deciding upon the value of a proposed improvement 
involving a considerable change of length in a road. 

Where the road is so situated and the saving in 
distance proposed is such that it would enable teams 
to make an additional trip per da\^ in the hauling of 
freight, the difference in cost of transportation is quite 
tangible and readily estimated; but where the traffic is 
of a more indefinite nature, or the saving proposed 
insufficient to admit of additional trips, the value of 
the difference of length depends upon the value to 
other work of the small portions of time of men and 
teams which may be saved by the shorter route — a 
value which exists, but is difficult to estimate. 

There is also a value in the saving of distance due 
to the advantage to the community of br'ng.ng t'.-C 
various points closer together, such as brmging two 



54 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

towns into closer relations or bringing country property 
nearer to markets. The method of considering the 
cost as proportional to the work done will therefore 
probably give a fair idea of the actual economy in any 
saving in the work of transportation. 

The value of reducing distance varies with the 
character of the road surface. As the cost of transpor- 
tation is less over a smooth than over a rough surface, 
on account of the lighter traction, the value of reduc- 
ing distance is also less on the smooth surface. 

The value of saving distance also is greater on a 
road where the ruling gradients are steep than upon 
one with light gradients, because of the greater num- 
ber of loads necessary to move the same traffic. 

The cost of maintenance of a road varies with its 
length, and under similar conditions may be con- 
sidered, like the cost of transportation, to be directly 
proportional to the length of road. 

The saving in cost of maintenance from decreasing 
distance must of course be added to that in cost of 
transportation in order to find the actual value of a 
change of length. 

The value of straightness for a country road is fre- 
quently very much overrated. Considerable devia- 
tions from the straight line may often be made with 
but slight increase in length, and there seems to be no 
good reason for insisting upon absolute straightness. 
The error is commonly made of sacrificing grade and 
expense in construction to the idea of straightness 
without the attainment of any considerable saving in 
length. 

It involves in many cases the injury of the beauty of 
the road and of the landscape, with no compensating 
economic advantages. 



LOCATION OF COUNTRY ROADS. 55 

Art. 18. Rise and Fall. 

B}^ the amount of rise and fall is meant the total 
vertical height through which a load must be lifted in 
passing in each direction over the road. It is distinct 
from and independent of the rate of gradient. 

The minimum amount of rise and fall is found 
where the rise is all in one direction and the fall in 
the other, each being equal to the difference of eleva- 
tion of the terminal points. Anj^ increase in the rise 
and fall bej^ond this amount is represented bj^ the rise 
encountered in passing from the higher to the lower 
terminus. This may be considered as avoidable rise 
and fall. If the cost of developing the work necessary 
to overcome rise and fall be the same as that of develop- 
ing an equal amount of work to overcome distance, the 
rise and fall may be evaluated in terms of distance, 
and any change in rise and fall may be considered as 
though it were a difference in distance and treated as 
in Art. 17. 

The value of rise and fall in terms of distance will 
depend upon the nature of the road surface, as the 
work necessary to lift a given load to a given height is 
a constant, while the work done in hauling a load over 
a given distance will vary with the resistance offered to 
traction by the surface. Thus, taking the surface as 
above, the work of lifting one ton through a rise of 
I foot is 2000 foot-pounds, while with a tractive force 
of 100 pounds per ton 2000 -^ 100 =20 feet, the 
distance a ton ma^^ be moved on the level surface in 
developing 2000 foot-pounds of work. Therefore I 
foot of rise or fall maj^ be considered as equivalent to 
20 feet of level distance, and the value of reducing the 
amount of rise and fall maj^ be found from that for 



56 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

reducing distance. If the road considered were a 
first-class macadam road, with resistance of 40 pounds 
per ton, I foot of rise or fall would equal 2000 ^ 40 = 
50 feet of distance. 

Where the rate of grade is less than the angle of 
repose of the wheels upon the road surface (see Art. 2) 
no additional work is imposed, by avoidable rise and 
fall, upon teams hauling loads over the road. The 
amount of work done in lifting the loads up the rise 
is equal to that done by the grade in diminishing trac- 
tion in descending the fall, and the total work required 
is equal to that necessary to haul the loads from one 
terminus to the other upon a uniform gradient. Upon 
an undulating road, therefore, where the grades are 
light, there is no economic advantage in reducing the 
rise and fall of the road. 

When the rate of grade is greater than the angle of 
repose, the amount of work imposed by avoidable rise 
and fall is equal to twice that caused by the excess 
of fall above that at the angle of repose. In this 
case an additional amount of work must be done in 
applying a resistance to prevent the too rapid descent 
of the vehicle in going down the grade. The amount 
of this work in any case equals the work done in lifting 
the load to a height equal to the difference between the 
actual rise of the grade in question and the rise of a 
grade of the same length and a rate equal to the angle 
of repose. Thus on an ordinary earth road whose 
resistance to traction where level is 1 00 pounds per 
ton, suppose a grade to occur of 8 feet per 100, 1 000 
feet in length. For the road surface we have 100 -^ 
2000 = .05, and the angle of repose is a 5 per cent grade. 
Then 8 per cent — 5 per cent = 3 per cent, or the 
brake-power necessary to secure uniform motion is 



LOCATION OF COUNTRY ROADS. 5/ 

the same as Avould be necessary to haul the load up a 
3 per cent grade, and a grade of 3 in 100 for 1 000 feet 
gives 30 feet. The work to be done in holding back 
the load for the looo-ft. grade is therefore the same as 
would lift the load through a vertical height of 30 feet, 
or the fall of 8 feet per 100 for 1000 feet has the same 
effect as 30 feet of rise in the same direction, pro- 
vided brake-power costs the same as animal power. 
The "work saved to the traffic passing down this grade, 
by eliminating it as avoidable rise and fall (without 
changing the ruling gradients), would be twice the above 
amounts or equal to lifting the loads through 60 feet 
of rise. 

Art. 19. Rate of Grade. 

The effect of anj^ change in the ruling gradient upon 
a road depends to a considerable extent upon what 
portion of the traffic may be carried in full loads. The 
lighter portions of the traffic are not so seriously 
affected b}^ heavy gradients as the heavy portions, 
although there is an advantage in light gradients for 
any driving. The rate of speed which ni3.y be emplo3"ed 
will be less upon the portions of the road having heavy 
grade, and the time occupied in a trip over the road is 
therefore affected somewhat by the rate of grade. 

The desirabilitj^ of a road for general driving is also 
much influenced bj^ the gradients employed, as is 
that value of the road which has for a basis the effect 
it may exert upon the attractiveness of the locality. 
These things all have a certain financial value, which 
of course it is quite impossible to estimate with any 
degree of accurac}', but which should be considered in 
determining the allowable maximum gradient in any 
case in practice. 



58 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

For heavy traffic, such as the transfer of goods from 
one town to another or the marketing of country prod- 
uce, the hmitation of load placed upon the traffic by 
the gradient is a matter of importance, the effect of 
which is calculable upon the cost of transportation. 
If in any case the approximate amount of this heavy 
traffic which is likely to be carried in full loads be deter- 
mined, the relative costs of its transportation over 
two lines of differing gradient, other conditions being 
similar, will be nearly proportional to the number of 
loads required to move the traffic over each gradient. 

In estimating the value of reducing the rate of grade, 
it may be considered, as in the case of a reduction of 
length, that its value to the community is represented 
by the saving in annual costs of transportation, and 
that the amount that may reasonably be expended in 
increased cost of construction to effect a reduction of 
gradient is the sum upon which this annual saving is 
the interest. 

The length of a road and the amount of rise and 
fall on it determine the amount of work that must be 
done in hauling a load over the road. The rate of 
gradient, on the contrary, does not affect the amount 
of work necessary to move the traffic, but it limits the 
load that a horse may haul at one trip. 

The establishment of a proper rate for the ruling 
grade of the line is, therefore, usualty the most impor- 
tant point in location. In localities where light gra- 
dients are easily obtained the problem of location is 
greatly simplified. 

By referring to Art. 3 the comparative loads that 
a horse may draw up different grades will give some 
idea of the importance of carefully considering the 
question of gradient. In nearly all cases in practice 



LOCATION OF COUNTRY ROADS. 59 

there is a considerable latitude within which gradients 
ma}^ be chosen. It is usually a question of heavier 
gradients as against greater distance and larger first 
cost for the road. It may be remarked that it is only 
under exceptional circumstances that it is either neces- 
sary or advisable to use a steeper gradient than 5 per 
cent on the new location of a countr}^ road of an}^ 
importance. Grades steeper than the ruling gradient 
may sometimes be introduced over short distances 
without impairing the efficiency of the road, as horses 
are usually able to exert for a short time a force much 
greater than they can continuously exert. If the 
length of grade be quite short, 200 or 300 feet, a horse 
can about double his ordinary power in passing it. 

^\^lere long steep grades must be used, it is desirable 
to break them b^^ short stretches of lighter gradients to 
provide resting-places for horses. 

Heavy gradients also have the disadvantage of 
retarding traffic in the direction of falling grade, and, 
as suggested in Art. 18, of requiring the expenditure 
of work to hold the load from too rapid descent. 

Art. 20. Examination of Country. 

For the purpose of obtaining the requisite data 
upon w^hich to base the location of a road, it is neces- 
sary that a careful examination be made of the topo- 
graphical features of the country through which the 
line is to pass. The relative elevations of the termini 
of the line and of intermediate points should be 
obtained, and the directions and steepnesses of the 
various natural slopes determined. 

If a line were to be located connecting points at long 
distances from each other, as sometimes occurs in 



6o A TEXT-BOOK ON ROADS AND PAVEMENTS. 

railway location, it would be necessary to study the 
general configuration of the country, noticing the 
direction of flow of the streams, and the location and 
elevations of the various passes in the ridges through 
which it might be possible to carry the line. Usually 
it would be found that the country is composed of a 
series of valleys, separated by ridges, branching in a 
systematic manner from the main watercourse of 
the region, and that the passes in the ridges occur at 
the head of side streams, and especially where streams 
flowing into valleys on opposite sides of the ridge have 
their sources near each other. 

In the location of common roads, however, the prob- 
lem is ordinarily of a less extended nature, and may 
consist in joining two points lying in the same valley, 
or in joining points in adjacent valle^^s by a line pass- 
ing over a ridge. In these cases it is only necessary to 
take into account the slope of the valleys in question, 
the positions and elevations of available passes, and the 
side slope of the ridges. 

The slope of the bed of a valley, in hilly country, 
usually forms a concave curve, the rate of slope gradu- 
ally increasing from the lower to the upper end. In a 
valley of considerable length this increase in the rate 
of slope may be very gradual or in short valleys rising 
to a considerable height it may be more sudden. The 
profile A BCD in Fig. ii shows the slope of a short 
valley which decreases in slope from about ten feet 
per hundred at the upper end to about two feet per 
hundred at the lower end. 

When a map of the countr3^ to be traversed is avail- 
able, showing the positions and elevations of the points 
controlling the location, the work is very much simpli- 
fied, the reconnaissance may for the most part be 



LOCATION OF COUNTRY RO.\DS. 



6i 




62 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

limited to a study of the map, and the routes may be 
sketched upon the map to be tried in the field. If the 
map at hand is an accurate contour map on a sufficiently 
large scale, the entire location may be worked out in 
detail upon the map, leaving only the work of staking 
out the line to be done upon the ground. 

Maps may be obtained, in most parts of this country, 
upon which the horizontal positions of points may be 
readily fixed with sufficient accuracy for the purposes 
of the preliminary examination. Where such maps are 
not obtainable, the positions of points must be ascer- 
tained and a rough map prepared. For this purpose 
directions may be measured with a pocket compass, 
and distances estimated or obtained by the use of an 
odometer or pedometer, as may be most convenient. 

Differences of elevation are easily obtained with a 
fair degree of accuracy by the use of an aneroid 
barometer, and slopes may be measured with a hand 
level. 

Where the rough means ordinarily employed in the 
reconnaissance are not sufficiently accurate to deter- 
mine the controlling points of the lines to be adopted, 
a more complete examination of the country may often 
be made by a rapid topographical survey b}^ means of 
the transit and stadia method. 

Whatever means may be adopted for doing the 
work, the preliminary examination should determine a 
map showing the approximate positions of the con- 
trolling points through which the road must pass, and 
enable a rough sketch to be made of the slopes of the 
country through which the line is to be run. 



location of country roads. 63 

Art. 21. Placing the Line. 

After the preliminary examination of the locaHt}^ is 
complete and the positions and elevations of the con- 
trolling points of the line are known with reference to 
each other, the line must be selected and run in upon 
the ground, or, if the reconnaissance is not conclusive 
as to the position of the best line, it is advisable to 
run in two or more lines and make a more detailed 
comparison between them. 

The controlling points of a line are those points at 
which the position of the road is restricted within 
narrow limits, and is not subject to change. These 
ma}' be points where the location is governed bj^ the 
necessity' of providing for traffic, or points where the 
position of the line is restricted b}' topographical con- 
siderations, such as a summit over which the line is to 
pass a ridge or a favorable location for a bridge. 

Where the hne is to be located to a uniform gradi- 
ent, it should be started from the controlling point at 
the end of the grade, which is usualh^ the summit. It 
is then laid off along the slope in such manner as to 
cause it to have continuously^ the rate of grade decided 
upon. Taking D (Fig. ii) at the summit of the vallej^ 
as the controlling point, it is seen that the distance 
from C to D is sufficient to give a gradient of 10 in 
100 b}' following directh' down the valley, and the line 
with that gradient may be run in that manner. 

The maximum gradient from A to C is, however, 
onh' 5 in 100, and if thought advisable the same maxi- 
mum gradient m.s.y be used between C and D b}^ run- 
ning the line DHC diagonally down the slope, as 
shown. This line, having one-half the gradient, will 
have about twice the length of the line CD. 



64 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

In running this line it is started from the highest 
point of maximum grade, and points at the surface of 
the ground are continually selected, in advance of the 
placing of the line, which are at the proper elevation 
to permit the grade to pass through them. This may 
be accomplished by setting off the angle of the gradi- 
ent upon the vertical circle of the transit, or upon a 
gradienter, and sighting upon a rod which is moved 
until the line of sight strikes it at the same height from 
the ground that the instrument is setting. The points 
for the line may also be found by running a line of 
levels ahead of the transit line (a hand level is conven- 
ient for this purpose) and pacing distances upon which 
to reckon the gradient, the distances and elevations 
being frequently checked upon those of the measured 
line. 

The location of a gradient upon a common road 
differs from that upon a railroad only in that steeper 
gradients are used, sharper curves or angles may be 
employed, and the gradients need not be lessened on 
ordinary bends or curves. If the line is to make a 
turn upon the slope as at H, the grade should be 
flattened at the turn, and a curve of as large radius as 
possible, without too great expense for grading, be 
introduced. 

In a manner similar to the above a line might be 
run from D on the other side of the valley, which 
using a 5 per cent gradient would give the line DML, 
reaching the bed of the valley at the point L. A lighter 
gradient may be obtained from A to D b3^ starting 
from D and going down by a continuous gradient of 
4 in 100 on the line DFGA, and greater or less rates 
of descent may be adopted and lines corresponding to 
them located, as may be considered advisable. 



LOCATION OF COUNTRY ROADS. 65 

The center-line for a final location should be care- 
fully run, and points permanently marked from which 
it may be relocated when necessary. An accurate line 
of levels should also be run over the center-line and a 
profile drawn, upon which the grades ma}^ be estab- 
lished and earthwork estimated. 

After placing the center-line, topography should be 
taken carefulh" upon each side of the line for some 
distance, and a map drawn showing the topograph}^ 
and giving elevations by means of contours. This will 
serve to show whether the line is placed to the best 
advantage, and whether an^^ changes are desirable. 
This is especially necessar3^ over rough ground or 
where the line is on maximum gradient, as frequentl3^, 
and perhaps usualh^ the first line run will be useful 
only as a preliminary^ line, which with its accompany- 
ing topography will permit a proper location to be 
made. 

Art. 22. CoMPARisox of Routes. 

In selecting a line for the construction of a road the 
principles already mentioned in the earh^ part of this 
chapter should be had in mind. The line must be well 
designed to accommodate the traffic. It should have 
as eas\" grades, short length, and small rise and fall as 
is consistent with a reasonable cost of construction, in 
order to give light costs for transportation and for 
maintenance. 

Suppose in the case shown in Fig. 1 1 that it is desired 
to connect the village at the point A with the point D 
and with the roads leading through the passes at F 
and /. Which line it will be the most advantageous 
to adopt depends upon the relative importance of the 
traffic to the various points considered. 



66 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

The shortest, and probably cheapest, line from A 
to D would be obtained by following the valley over 
the line ABCD, which line, as shown by the profile, 
would give a maximum gradient of lo in 1 00 between 
C and D. The line FB joining the first line at B 
would afford communication with the summit at F 
with a maximum gradient of 5 in 100. If the traffic to 
the point D be small and unimportant, so that addi- 
tional expense in reducing the gradient from C to D 
is unadvisable, these lines might prove a satisfactory 
location. 

If, however, D be a point of importance and the 
traffic from A to D heavy, it will be necessary to adopt 
some means to reduce the gradient from C to D. 
Leaving out of consideration the point F and consider- 
ing B and C as points of minor importance, it might 
be advisable to use the line ALMD with a uniform 
5 per cent gradient from D to L, and branches to 
connect with C and B. This would give a line but 
little longer than the valley line, with only one-half the 
ruling gradient of that line. 

If C is not important and can be neglected while B 
and F must be considered, the line ABEHD has a 
maximum gradient of 5 in 100, and connects A with 
the points BF and D with a minimum total length of 
road (being less than the valley line first considered). 

When B and C must both be considered as of im- 
portance as well as F and D, the lines ABC HE and 
HD will give a ruling gradient of 5 in 100 to both F 
and D, and passing through B and C with a somewhat 
longer line than in the last case. 

This arrangement would make the length of haul 
from A to D and F each longer than bj^ the first line 
considered; but the gradient to D would be lighter, and 



LOCATION OF COUNTRY ROADS. 6j 

the total length of road to be constructed and main- 
tained would be less. 

In case the points B and C are both unimportant, 
and the line through the valley may be neglected, the 
line AGFD provides a ruling gradient of 4 in 100 from 
A to both F and D, and connects them with each 
other, with about the same length as the shortest 5 per 
cent gradient. When the point / must be taken into 
account, this line may be connected with / by the line 
GI having a gradient of 4 in 100. This would give the 
shortest line of uniform gradient to connect A with the 
three points I, F and D, and possibly a desirable line to 
construct when the line through the point / is impor- 
tant, even if the valley road from A to B is also neces- 
sary. 

The lines upon the side slopes are usually more ex- 
pensive to construct than the valley lines, and the dif- 
ferences of first cost of the various lines must of course 
be considered. The importance of a difference in ex- 
pense of construction depends upon the traffic to be 
hauled over the road and the kind of surface to 
be used. Where a broken-stone or gravel road is to be 
constructed at considerable expense, the difference of 
cost due to a change of location is relatively less 
important as being a less percentage of the whole cost, 
while the difference of tractive effort due to grade is 
of more importance, as being a higher percentage of 
that upon the level, than w^ould be the case with an 
ordinary earth road. 

As is easily seen from the above the choice of a 
location for a road, while depending upon principles 
easily stated, is in reality a matter requiring the use of 
judgment, and is not readily reducible to a financial 
comparison stated in money values, because the data 



68 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

concerning the volume of the traffic and the cost of 
conducting it can be determined only very roughly, 
and contains many elements of error. For purposes of 
comparison to aid the judgment, approximate data 
may often be assumed or determined by a study of the 
localities affected. In some cases observations may be 
made of the number of teams of different classes pass- 
ing certain points within certain times, to give a basis 
for estimation of the annual volume of traffic. In 
other cases, the annual hauling traffic, which is usually 
the most important portion of the traffic in considering 
location, may be estimated from the known interests of 
the locality. Thus, if the produce of a certain section 
of farming country must be hauled over a given road 
to market, the amount of this produce may be esti- 
mated from the acreage, and the relative number of 
loads upon different grades then determined.. The 
cost per load over the road would then need to be as- 
sumed in order to find the annual value of a reduction 
of grade. 

In the same manner, the effect of changes of length 
and in the amount of rise and fall may be found as 
indicated in Arts. 17 and 18. 

All of these items must be combined to find the rela- 
tive total costs of transportation for each route. The 
cost of construction and of maintenance for each line 
must then be estimated, and that line is the most ad- 
vantageous which makes the sum of the annual charges 
and the interest on the first cost a minimum. Where 
several lines of traffic are to be considered together as 
in Fig. II, the cost of conducting all of the traffic by 
each system of lines that may be emplo3^ed must be 
considered, the entire cost being made a minimum for 
the system to be adopted. 



LOCATION OF COUNTRY ROADS. 69 

Art. 2:^. Changing Existing Locations. 

The problem that arises oftener than any other 
in country-road location is that of improving short 
stretches of road, where, owing to defective location, 
the grades are mmecessarily heav3^ the length unneces- 
sarily great, or the ground over which the road may 
pass such as to make its maintenance in good con- 
dition difficult and expensive. The first of these is 
the most common defect of ordinary country roads, as 
shortness of distance has very commonly been obtained 
by the disregard of the desirability of light gradients, 
which in very many cases are easity obtainable. 

The principles to be observed and methods of pro- 
cedure in making the new location are exactly the 
same as in an original location, save that in this case a 
road already exists, and the question of economy is one 
of determining whether the advantages to be gained 
in lessened costs of transportation and maintenance 
is sufficient to warrant the expense of obtaining new 
right of waj^ and constructing new road. 

In Fig. 12 is given an example that is frequently met 
in practice, where the existing road abed runs over the 
point of a hill, with heavy gradient, while a line of very 
much lighter gradient might be located around the 
base of the hill through the pass at e, giving a greater 
length of road, but much less rise and fall. The line 
bed in the figure has a length about 800 feet greater, a 
rise and fall 70 feet less, and a maximum gradient one 
half as steep as the line bed. These relations are shown 
in the profile in Fig. 12. 

If the road in question be a common earth road, 
I foot of rise and fall may be taken as equivalent, in the 
work required to haul a load over it, to 20 feet of dis- 



70 A TEXT-BOOK ON ROADS AND PAVEMENTS. 




LOCATION OF COUNTRY ROADS. 7 1 

tance, and the 70 feet saved b3' the new location would 
be equivalent to 1400 feet of distance. Hence, the 
line bed may be considered as having an equivalent 
length for purposes of traffic 1400 — 800 = 600 feet 
shorter than the line bed. In addition to this, loads 
may be taken over the new line in direction b to d 
more than double, and in direction from d to b triple, 
in weight those that can be taken b^^ the same power 
over the old line. 

A further improvement of the line Yn3.y also be 
possible, if the new line can join the old one at a point 
lower down than b, b3^ running a lighter gradient than 
5 in 100 from the point e. Thus the line efa would 
give an uniform gradient of 4 per cent, but w^ould 
require the construction of more new line. 

In considering changes of location, it is also neces- 
sary to take into account the interests of adjoining 
owners. Houses and buildings are largely located with 
reference to the existing position of the roads, and 
changes in the position of a road may involve injury to 
such property. The question then becomes largely 
one of sacrificing the interests of the users of the road, 
or those of the adjoining owners — a question that 
should be, but commonly is not, decided by consider- 
ing what will be of most advantage to the general com- 
munity. 



CHAPTER IV. 

IMPROVEMENT AND MAINTENANCE OF COUNTRY ROADS. 

Art. 24. Nature of Improvements. 

Ordinary country roads may be classified as earth 
roads, gravel roads, and broken-stone roads. The 
larger number of common roads throughout this coun- 
try belong of necessity to the first class. In a few of 
the more enterprising communities the more important 
roads are constructed of gravel or broken stone. 

The percentage of roads of the better class is, how- 
ever, very small, and although there has recently been 
a distinct improvement in this particular, the inability 
of rural communities to at once raise the funds neces- 
sary for the general construction of first-class new 
roads will cause their increase to be very gradual. 

Improvement in country roads may be of several 
kinds: 

(1) Changes in location, by which better alignment 
or better gradients may be obtained, or by which the 
natural conditions of surface or drainage may be im- 
proved. This has been discussed in Chapter III. 

(2) Reconstruction of the road-bed, as in regradmg 
steep slopes to give lighter gradients, or in raising the 
road-bed across low and wet places to provide for 
drainage. 

(3) The construction of artificial drainage where a 
road is built over ground which is likel^^ to become soft 
in wet weather, or where water may reach the road-bed 

72 



IMPROVEMENT OF COUNTRY ROADS. 73 

from underground sources. This has been discussed in 
Chapter II. 

(4) Improvement of the surface, which may consist 
in re-forming the surface of natural earth, or in the 
construction of an artificial surface or pavement, the 
latter of which will be discussed in separate chapters. 

The more important lines of travel leading out from 
the towns will gradually be improved by the con- 
struction of broken-stone or other permanent roads, but 
this constitutes but a small percentage of the total 
mileage, and the problem in common-road improve- 
ment is for the most part that of making the most of 
the roads that exist, rather than reconstructing them 
with new^ material. The materials and fmids imme- 
diately available must be used to secure as much im- 
provement as possible. 

Earth roads, under the most favorable conditions, 
do not usually attain a high degree of efficiency, and 
are not economical under any considerable traffic. 
They are, however, capable of much improvement and 
need not become, as they frequently do, practically 
useless during a large portion of the year. This im- 
provement must be gradual and come about through 
the adoption of more rational methods of maintenance, 
rather than through immediate reconstruction of the 
road surfaces. 

Art. 25. Grade and Cross Section. 

As already explained in Art. 10, the drainage of the 
surface of a road is accomplished by crowning the sur- 
face and giving it a proper longitudinal slope. Under- 
drains will not drain water from the surface of the road, 
and unless the crown is at all times maintained and the 



74 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

surface kept smooth, water is likely to stand upon the 
surface and soften it. 

Grade. The surface of a country road should not be 
level in the direction of its length, but should have a 
sufficient longitudinal slope to drain any water from 
its surface which might otherwise be held by small ruts 
or depressions. The minimum grade for an earth 
road should be at least ^ foot in 1 00 feet, and for a 
broken-stone road nearly as much. The grade, except 
in very rough country, should not exceed 4 or 5 feet 
per 100, and when steeper grades are necessary, they 
should be made as light as may be feasible. The effect 
of changing the rate of grade is discussed in Art. 19. 

Width. Too great width of roadway upon country 
roads causes an unnecessary expense in the con- 
struction and maintenance of the road, and the width 
should be only sufficient to provide space for the 
easy conduct of the traffic. For roads of ordinary 
traffic, this requires only that there be room for teams 
moving in opposite directions to freely pass. A 
width of 20 feet is ample for most country roads, and 
for roads of lighter traffic 16 feet is often sufficient. 
Outside of this width, side ditches must be formed for 
carrying the surface drainage. 

In the construction of gravel or broken-stone roads, 



TTTTTm/t 



Fig. 13. 

the paved portion of the road does not usually extend 
to the full width of the roadway, a shoulder of earth 
being left on each side, as shown in Fig. 13. The width 
of broken stone on ordinary country roads may vary 



IMPROVEMENT OF COUNTRY ROADS. 75 

from about 12 feet to 1 6 feet. A greater width than 
this need only be employed on important roads which 
convey large traffic, or on city streets. 

While the improved portion of the road should be 
as small as is consistent with the proper discharge of 
the duty required of it, the available right of way need 
not be so restricted, but should be laid out wide 
enough to permit of the widening of the used portion 
when necessary, and allow room at the sides for pedes- 
trians, with ^ a grass border and line of trees. When 
trees are planted along the roadway they should not 
be placed so as to form a dense shade over any portion 
of the traveled road, although a moderate shade is 
not a disadvantage, and care should be used that they 
are not near enough to a covered drain to permit the 
roots to grow into the drain and choke it. 

Crown. The surface of a road must be crowned 
sufficiently to cause the water which falls upon it to 
run at once into the gutters. The height of crown 
required depends upon the character of the surface and 
upon the grade of the road. A high crown is objec- 
tionable because it concentrates the travel in the middle 
of the road, which tends to wear hollows longitudinally 
along the road into which water may settle; but if 
the crown be too low, small depressions worn into the 
surface by the traffic may hold water and cause the 
road to become soft. The slope from the center to 
the side of an earth road should not be less than one 
in twenty nor greater than one in ten, corresponding 
to a height of crown from one-fortieth to one-twentieth 
of the width of the road. For roads upon which the 
surface can be kept in smooth condition and on 
moderate grades, the lower limit may be used, but on 
the average country road a steeper slope is desirable 



76 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

and a crown of one-twentieth the width is not too 
great to secure efficient drainage. 

Gravel and macadam roads should be crowned 
somewhat less than earth roads. On wide macadam 
streets kept in smooth condition^ the side slopes may 
be as low as one in thirty, or a crown equal to one- 
sixtieth of the street width. On such work, however, 
it is more common and probably better to use a crown 
of one-fiftieth, or even one-fortieth the width; while on 
ordinary graveled or macadamized country roads the 
crown should be from one-thirtieth to one-twentj?-- 
fourth of the width. When constant attention and 
careful maintenance can be relied upon to keep the 
surface in smooth condition a less crown may be used 
than would be allowable if the road is likely to be 
subjected to considerable wear between periods when 
repairs are made. 

Form of Section. There is considerable difference of 
opinion amongst road builders as to the best form to 
give the surface of a road. Some use a section com- 
posed of two planes of equal inclination rounded off in 
the middle and sloping uniformly to the sides as shown 
in Fig. 14. Others prefer to use a convex curve. 



Fig. 14. 

approximately the arc of a circle, or more commonly, 
a parabolic curve, which is practically identical with 
the circular arc. The exact form is not a matter of 
importance on a country road, and either of them, or 
some intermediate form, may give good results in 
practice. It is not desirable to insist upon great 



IMPROVEMENT OF COUNTRY ROADS. 



77 



accuracy in the form of section provided a proper 
crown be given and the surface be properly smoothed. 
Where a smooth pavement is used, however, it is 
desirable to place it accurately to a uniform section^ 
Gutters. At the side of the road longitudinal ditches 
must be provided for the purpose of carrying the water 
drained from the surface of the road to some point 
where it may be turned into a natural drainage channel. 
In many instances these side ditches also carry the 
drainage from land adjacent to the road. The size 
and form of the gutters will naturally depend upon the 
quantit}^ of water to be carried and the slope of the 
gutters. In some instances the extension of the road 
surfaces, as shown in Fig. 13, will be sufficient and no 
special gutters will be required. In deep cuts where 
the excavation necessary to form side ditches would 
be expensive, a tile may be placed under each side of 
the road, as shown in Fig. 16, into which the drainage 




Fig. 16. 



from above the cut and from the small gutters may 
be carried. 

Broad, shallow gutters are, in general, to be pre- 
ferred to deep and narrow ones. The side slopes 
should not, in any case, be less than 2 horizontal to 
I vertical, and 4 or 5 to I, on the side next to the road- 
waj^ is better. Shallow gutters are easier to form and 
keep clean, and are not so likely to wash out at times 
of heavy rainfall. It is not desirable to use deep side 



7^ 



A TEXT-BOOK ON ROADS AND PAVEMENTS. 



ditches for the purpose of under drainage, and water 
will not be drawn from the surface of a hollow roadway 
into such ditches. Fig. 17 shows a common form 
where it is intended to use the side ditches to prevent 
any seepage of water from the sides to the road-bed. 




Fig. 170 

This is the standard section given for state aid roads 
in New York, using a ditch two feet deep with side 
slopes two to one. This form is also used by the 
Massachusetts Highway Commission, who recommend 
ditches 3 feet deep, I foot wide at bottom and with 
slopes 2 to I, Fig. 18 shows section recommended 
by the Illinois Highway Commission for use in level 




Fig. 18. 



country, where the roadway is formed by material 
excavated from the side ditches. 

On the average country road, surface drainage will 
be amply secured by gutters 18 inches to 2 feet below 
the crown of the roadway, and side ditches of greater 
depth are a source of unnecessary expense. Where 
under drainage is necessary it should be accomplished 
by tile or other covered drains. 



IMPROVEMENT OF COUNTRY ROADS. 79 

Art. 26. Earthworks. 

Improvements to the road-bed of an existing coun- 
try road ma3^ have for their object the reduction of 
gradient upon steep inclinations by cutting the material 
from the road-bed and lowering the surface of the 
road on the upper part of the grade, and filling in 
correspondingly on the lower part, or thej^ may be 
intended to provide better drainage by raising the 
road across low ground. 

In the construction of new roads, the formation of 
the road-bed consists in bringing the surface of the 
ground to the grade adopted for the road. This grade 
should be carefully established upon an accurate pro- 
file of the line, in such manner as to give as little 
earthwork as possible, both to render the cost of con- 
struction low, and to avoid unnecessarily marring the 
appearance of the countr^^ in vicinity of the road. 
The most desirable position of the grade line is usually 
that which makes the amounts of cut and fill about 
equal to each other, especially where room for borrow- 
pits, or spoil-banks, would be expensive, and it is 
desirable to make the embankment for the most part 
of the material taken from the road excavations. On 
side-hill work, one side of the road is commonly in cut 
and the other in fill, and where the side slopes are 
steep, it is usually better to make the road mostly 
in cut on account of the difficulty of forming stable 
embankments on steep ground. In balancing cuts and 
fills, it is necessary to estimate the quantities for the 
full width, including side ditches, as the grade should 
be placed high enough to permit using the material 
cut from the ditches in the embankment. 

Shrinkage. Earth, in embankment^ will compact 



8o A TEXT-BOOK ON ROADS AND PAVEMENTS. 

closer than it is found in its natural state, and allow- 
ance for shrinkage must be made in estimating the 
amount of excavation necessary to form a given 
embankment. On an average, ordinary soil may be 
expected to shrink lO to 12 per cent of its bulk; gravel 
or sand will shrink a little less than this, 8 or 9 per cent; 
light surface soils a little more, 14 or 15 per cent. 
The shrinkage may also be somewhat affected by the 
method of construction used in forming the embank- 
ment, being slightly less for work placed by wagons 
than for that by scrapers, and still less for wheelbarrow 
work. 

Settlement. In forming an embankment, allowance 
must sometimes be made for subsequent settlement, 
by raising the top of the embankment above the 
required grade. Where scrapers are used, the earth 
will usually be well compacted in placing, and no 
allowance is necessary; with dump carts or wagons the 
compacting is not so thorough, and a small allowance 
should be made; while when wheelbarrows are used or 
the earth is thrown into place with shovels, an allow- 
ance of 10 or 12 per cent must be added to the height 
of the embankment, in order to allow for the final 
shrinkage. Rock occupies more space in embank- 
ment than in excavation, and does not need allowance 
for shrinkage. 

Embankments. When embankments are to be con- 
structed, brush and weeds should be removed from the 
site and at points where the filhng is thin, it is desirable 
to remove all vegetable matter and soft material, to 
prevent unequal setthng and the formation of soft and 
spongy places in the surface of the road-bed. 

In constructing embankments across wet and 
unstable ground, it is frequently necessary to form an 



IMPRO\'EMENT QF COUNTRY ROADS. 8 1 

artificial foundation upon which to place the earth- 
embankment. This ma}^ be accomplished in some 
cases by excavating a little of the soft material and 
substituting sand or gravel, or in other cases it may be 
advisable to employ laj^ers of brushwood or fascines 
as a support for the enbankment. Sometimes it may 
be possible to drain the soft material by deep ditches, 
so as to render it capable of sustaining the road, and 
in all cases drainage should be provided in so far as 
possible to make the embankment more secure. 

When embankments are to be found on sloping 
ground, the surface of the ground should be stepped 
off, in order to hold the earth-filling from sliding upon 
the natural surface at the line of contact between the 
two, until it becomes sufficiently settled for the develop- 
ment of cohesion to cause it to become one solid mass. 

In many cases where roads are to be constructed 
along steep slopes, it is found cheaper to use retaining 
walls to sustain the road upon the lower side and the 
earth cutting on the upper side than to cut long slopes 
or form high embankments. 

Catch-water drains are necessarj^ on the natural sur- 
face above the top of all high slopes in cuttings to 
prevent the surface water from washing down and 
destroj^ing the face of the slope. 

Where springs are tapped by a cutting, drains must 

^■e nrovided to remove the w^ater without injurj^ to 

" : ^lope; and where the subsoil may become wet in 

'■' weather, it may be necessary to provide sub- 

""-^e drains along the slope to prevent the earth 

")n"ng saturated and sliding down into the roadway. 

Slooes, both of excavation and embankment, are 
^^eath^ improved by being sodded or sown with grass. 
This aids in the maintenance of the slopes, by render- 



82 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

ing them more capable of resisting the abrading action 
of such water as falls upon them. It also greatly 
improves their appearance. 

The most important principle involved in the forma- 
tion of a road-bed, which should be always in mind, is 
that earth, in order either to sustain a load or to main- 
tain a slope, must be kept dry, or at least prevented 
from becoming saturated with water, as both the 
cohesive and frictional resistances of earth are dimin- 
ished or destroyed when it becomes wet, and it is also 
then liable to the disturbing action of frost. 

Methods of Handling Earth. In the grading of roads 
or streets, the earth is commonly moved by scrapers, 
or wagons, after being loosened by plowing. For 
ordinary work the common railroad plow is used, drawn 
by two, or in hard material four, horses. In breaking 
up very hard material, like an old gravel surface, a 
rooter plow may be needed with four or six horses. 
Economical handling of the material requires that it 
be well loosened and the plowing is usually but a small 
part of the cost. 

For moving the loosened earth, drag scrapers may 
be used for short hauls; they are economical for 
distances up to about 1 00 feet. For distances greater 
than about 80 to 100 feet wheel scrapers will be more 
economical; for the shorter hauls, the small (number i) 
scraper, with a single team to handle each scraper; 
for longer hauls, above 200 to 300 feet, the larger 
(number 3) scrapers, with snatch teams to load them. 
For hauls greater than about 500 to 600 feet, wagons, 
loaded by men with shovels, will usually be cheaper 
than scraper work. 

In flat country where the grades conform closelj^ to 
the natural surface, and the road-bed is formed with 



IMPROVEMENT OF COUNTRY ROADS. 83 

earth taken from the side ditches, the use of the eleva- 
ting grader is usually economical and frequently makes 
possible the construction of the road at very low cost. 
This consists of a frame resting upon four wheels, 
from which is suspended a plow and a wide traveling 
belt. The plow loosens the earth and throws it upon 
the incHned belt, which carries it to one side and de- 
posits it near the middle of the road. The ordinary 
machines are built to deliver the material at about 14 
and 17 feet horizontally from the point at which it 
is excavated. They are usually operated with eight 
horses. 

The ordinary road machine, or scraping grader, is 
also a convenient tool for this kind of work, and when 
the amount of material to be moved is small, and the 
work consists in cutting shallow side ditches and form- 
ing a road surface with material taken from them, is 
usuall}^ the most economical tool to employ. 

Work done by scrapers will usually be left in rough 
and lump3^ condition. For smoothing the surface, 
after the earthwork is roughly completed, the scraping 
grader or some form of road leveler may be used. For 
this purpose the blade is set so as to cut off the tops 
of the ridges and lumps, and fill up the hollows, with- 
out carrj^ing along any earth. 

Work Required in Moving Earth. The work 
required for moving earth under approximately the 
same conditions differs widely in practice. It depends 
upon the character of the material, the methods 
adopted for the work, the kind of labor available, 
and, most important of all, the skill with which it is 
managed. 

Loosening. In ordinary compact soil a plow and 
team with driver and plow holder will loosen 30 to 40 



84 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

cubic yards per hour. If the material be very hard, 
an extra team and man will be required for about the 
same, or a little less, amount of earth; while in hard 
buckshot, or in breaking up an old road surface, not 
more than one-half the amount may be loosened. 

Drag Scrapers. On short hauls up to about 50 
feet, a team with drag scraper may handle from about 
4 to 7 cubic yards per hour, and one man will load 
for 2 or 3 scrapers, depending upon the distance. 
If the lead be 100 feet, the drag scraper should move 
from 3 to 5 cubic yards per hour, and one man should 
load about 4 scrapers. 

Wheel Scrapers. Small (number i) wheel scrapers 
have a capacity of about ^ cubic yard of compacted 
earth, and for a haul of 100 feet may be expected to 
give about the same results as drag scrapers. For 
longer hauls, about one minute of time of team and 
driver will be required per trip for each 100 feet of 
additional distance, or about 5 minutes for each yard 
of material moved. 

The large size (number 3) wheel scraper may be con- 
sidered as carrying J cubic yard at a trip. A snatch 
team and extra man, or two extra men, will be required 
to load. These will load a scraper in an average of 
from one minute to two minutes. Two or three min- 
utes may be allowed for loss of time of scrapers on each 
trip in loading and unloading, and one minute for 
each 100 feet of haul. Thus, with a lead of 300 feet, 
a trip would be made in five or six minutes; 10 or 12 
trips per hour, or from 3 J to 4 cubic yards per hour 
for each scraper. 

Wagons. Over ordinary earth roads a team and 
wagon will carry an average load of I cubic yard; on 
good hard earth roads i^ yards may be taken. In 



IMPROVEMENT OF COUNTRY ROADS. 85 

loading ordinal}^ soil which has been loosened by plows, 
men may be expected to average from i^ to 2 cubic 
3^ards per hour. When the w^ork is fairly well organized 
and as many men are employed in loading as can con- 
venientl}^ work about a wagon (usuall}^ about 7 or 8) 
the loss of time of each team in loading, unloading, etc. 
may average about 5 to 7 minutes for loads of one cubic 
yard, while the time occupied in hauling will average 
about one minute for each 100 feet of lead. WTiendump 
wagons are used, about one minute would be saved 
on each trip. 

In estimating the cost of earthwork, about 20 to 25 
per cent should be added to the labor cost for con- 
tractor's profit, contingencies, etc. The skill with 
which earthwork is managed has much to do with the 
cost. Failure to property organize and systematize 
the work may easily increase the labor cost 50 per 
cent. It is not uncommon to find that two pieces of 
work identical in character, and conducted under the 
same conditions, differ 25 per cent in cost, because of 
the difference in the foremen handling the work. 

Art. 27. Earth Roads. 

The maintenance of an earth road surface in good 
condition consists in keeping it crowned and smoothed, 
so that water which falls upon the surface flows away 
immediateh^ into the gutters without remaining upon 
the road long enough to do serious harm in softening it. 
If ruts and depressions are allowed to form in the road 
surface, the3^ will hold water until it is absorbed into 
the road or evaporated, thus softening the road so 
that wheels will cut deeply into it, and gradually 
destroy its firmness. 



S6 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

The improvement of an earth road surface which is 
not in good condition must, therefore, be effected 
mainly by reshaping it into a form with proper crown 
to shedthe water. Its subsequent maintenance requires 
that it be frequently smoothed to prevent the forma- 
tion of ruts. It is practically impossible to maintain 
an ordinary earth road in good condition by the 
method of annual repairs. Where this method is 
followed, the road is usually shaped up with a road 
grader, after it dries sufficiently in the spring, and 
may present, a good surface during the summer and 
fall. It will, however, be worn hollow by the time 
bad weather sets in, and will be in condition to hold 
water and become saturated by heavy rains or melting 
snow. 

Shaping Section. The form of cross section which 
should be used has already been discussed in Art. 25, 
and the drainage of the road should be provided for 
where 'necessary as described in Chapter II. For 
cleaning the side ditches and forming the surface of 
the road, the ordinary road grader, or scraping grader, 
is used as mentioned in Art. 26. In this work, the 
blade of the grader is set so as to carry the material 
from the ditches toward the middle of the road, and 
repeated trips are made until the proper crown is given 
to the road surface. In doing this, care should be 
taken not to leave a ridge of soft material at the middle 
of the road, but to spread it evenly so that travel may 
take any part of the road surface, and thus compact it 
evenly. This may be accomplished by slightly raising 
the end of the blade of the grader nearest the middle 
of the road. In using the grader, the amount of 
material moved and its distribution are controlled by 
changing the angle at which the blade is set, and the 



IMPROVEMENT OF COUNTRY ROADS. 8/ 

elevation of its ends. Experience in handling the 
machine is necessary to its skillful use, and the amount 
of work required in forming a road is largel}^ dependent 
upon the skill and experience of the man operating 
the machine. Good results in such work also require 
that the teams used be well broken to the work. 

Where roads are shaped in this manner in the spring, 
the work should be done before the surface has become 
dry and hard, and while the earth is in condition to 
pack and unite with the surface upon which it is placed. 
After the ground has become dry and hard, the work* is 
more difficult and expensive, and the road is usually 
left in bad condition because the material moved, 
being hard and lumpj^ does not pack readil}^ under 
travel. 

Smoothing Surface. The maintenance of an earth 
road in good condition requires that surface be fre- 
quently smoothed so as to prevent the formation of 
ruts, which ma^^ hold water w^hen rain comes. Repair- 
ing the road by reforming the surface when it has 
gotten out of shape may improve it so that it will 
remain in fair condition so long as weather conditions 
are favorable, but when rain comes the surface will be 
softened so that wheels cut in to a small depth, making 
small indentations. These, if allowed to remain, will 
hold water at the next rain, causing the road to become 
soft to a greater depth and deeper ruts to form. If, 
however, after each rain the road be smoothed out, 
eliminating the ruts, and moving a little earth toward 
the middle of the road to replace that lost through 
wear, the road surface will be hardened and improved 
at each treatment, and will not retain water when con- 
tinued rains come upon it. 

The smoothing of the road surface should be done 



88 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

when the road is drying out after a rain; when it is 
not too muddy, but before it has become hard. The 
earth is then in condition to pack readily under travel, 
and will form a smooth hard surface when it becomes 
dry. If undertaken when the surface is too wet, it 
may be muddy and sticky to work; after it becomes 
dry, no good can be accomplished by working it, as it 
will not pack smooth and hard. When a road is kept 
shaped up by smoothing it after each rain, the earth 
composing the road surface becomes puddled, through 
being worked while wet, until it becomes practically 
impervious to water and forms a very hard crust on the 
surface. This effect is observed upon all soils which 
soften upon absorbing water, and become hard when 
dry, but is most noticeable upon clay or other heavy 
soils. The soil which makes the worst and most 
sticky mud when allowed to become saturated with 
water makes the hardest and most impervious surface 
when well maintained. 

Methods for Smoothing Surface. Several methods 
have sometimes been employed for smoothing the 
surface of an earth road. For the purpose of smoothing 
out the ruts in the spring, when a muddy road is drying 
up, a railroad rail 12 to 1 6 feet in length has sometimes 
been used, the rail being drawn by teams hitched at 
the ends so as to cut off the ridges and fill the ruts. A 
heavy stick of timber faced with steel on one edge has 
also been used in the same way. These methods may 
prove quite efficient at times when the roads are in bad 
condition, causing the surface to dry smoother than 
would otherwise be the case. 

The scraping grader is frequently used for light 
trimming of the surface, but is not usually an economical 
tool to use unless heavier work is to be done, on account 



IMPROVEMENT OF COUNTRY ROADS. 89 

of the weight of the machine and the cost of operating it. 
Several types of light road scrapers, or road levelers, 
as they are usually called, requiring only a single team 
and driver, of perhaps also a man to operate the 
machine, are occasionally used for this purpose. These 
levelers are sometimes mounted upon two wheels, and 
the blade made adjustable in position; others are simply 
cutting blades which slide upon the ground in fixed 
position. They frequentlj^ do good work in smoothing 
the surface when the soil is in proper condition, 
although thej' do not pack the material upon the 
surface of the road. 

Road Drag. The cheapest and most successful 
method yet devised for maintaining the surface of a 
road in good condition is bj^ the use of the road drag. 
This method has been used with great success in the 




Fig. 19. 

states of the Mississippi valley, where the maintenance 
of earth roads in condition to be used at all seasons 
had previously been considered an almost hopeless task. 
Its introduction is largelj^ due to the efforts of Mr. 
D. Ward King, of Maitland, Mo., who spent most of 
his time for a number of years in introducing and 
explaining his method of using the "split log drag.'* 
Fig. 19 shows a drag as commonh" constructed of split 



90 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

logs. The following description of the drag and its 
operation is taken from the report of the Illinois 
Highway Commission for 1906: 

"'The log should be from 10 to 12 inches in diameter 
and about 9 feet long. The holes in the front half of 
the log should be bored so that a slight slant forward is 
given to the lower part of the front face of the split log. 
The holes in the rear log are bored so that its flat face 
will be perpendicular to the sticks forming the con- 
necting braces which should be tapered at the ends 
so that they will fit snugly into the holes bored into the 
logs. The holes should not be less than two inches in 
diameter. The ends of the cross sticks should be 
split and wedges driven so as to secure the cross braces 
in place. The wedges should be driven crosswise of 
the grain of the log or plank so as not to split it. A 
diagonal cross brace is placed between the logs at the 
leading end to stiffen the frame of the drag. The 
distance from the face of the back log to the face of the 
front log should be about three feet. The lower front 
edge or toe of the drag should be protected by a strip 
of old wagon tire, or other piece of iron, about a quarter 
of an inch thick and 3 or 4 inches wide and about 4 
feet long. This strip of iron should be bolted to the 
front log and the heads of the bolts countersunk. The 
strip of iron should not be carried the entire length of 
the front log. 

"Chains should be provided with which to haul the 
drag, arranged with a short and long hitch as shown in 
the sketch, so that the drag will travel at an angle of 
about 45 degrees with the direction of the road. It 
will be noticed from the sketch that the long hitch of 
the chain goes over the log around one of the cross 
pieces rather than through a hole in the front log. 



IMPROVEMENT OF COUNTRY ROADS. 9 1 

This allows the earth to slide unobstructedly along the 
front face of the drag. '' 

The drag ma}^ also be made of planks, instead of 
logs; 2 by 12 inch planks are used for this purpose, 
reinforced on the inner side by 2 inch by 6 inch strips, 
to provide a greater thicloiess of wood through which 
to bore the holes. 

" When the road drag is properly used it spreads out 
the la3^er of impervious soil over the surface of the road, 
filling up the ruts and hollows until a smooth surface is 
secured. As a small amount of material is alw^ays to be 
pushed to the center, a slightly rounded effect will be 
given to the road, which may be increased or decreased 
as desired by subsequent dragging. By forcing the 
mud into the hollows and ruts, it is evident that the 
water must go out, which it does by running off to the 
side of the road. The drjnng out of the road is thus 
much facilitated and the road is made immediately 
firmer because the water is squeezed out. 

*'The effect of traffic over the road tends to press 
down and thoroughly compact the top of the road and 
each thin layer of puddled earth which the drag 
spreads over the surface every time it is used. After 
the first few draggings it will be noticed that the road is 
becoming constantly smoother and harder so that the 
effect of a rain is scarcely noticeable, the water running 
off the smooth hard surface which absorbs but little of 
it.'' 

The action of the drag differs from that of an ordi- 
nary scraper or leveler in that it packs the material 
upon the surface, while the leveler merely smooths the 
road by trimming off the high places and distributing 
the material into the low places. 

Constant attention is necessary to maintain an 



93 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

earth road in good condition. The dragging should 
be done as soon after each rain as possible^ and at all 
seasons of the year. If dragging is done before a 
cold spell, the road will freeze in a smooth condition, 
and will be in good condition when the frost leaves it. 

The amount of work required in dragging roads is 
comparatively small. While frequent attention is 
necessary, the work to be done at one time is insignifi- 
cant. In several instances it has been found that the 
cost of maintenance by dragging is much less than the 
expenditure previously incurred for shaping up the 
road with the grader in the springe 



SAND ROADS. 

When a road surface is composed of sand, it will be 
more firm when the sand is damp and more unstable 
in dry weather. Such roads require very different 
treatment from those on clay or loam. No attempt 
should be made to round up the road, as it is an advan- 
tage to retain as much moisture as possible. If clay 
is available, it is desirable to mix clay with sand in 
the road surface. If clay is put upon the surface when 
the materials are damp, the traffic will mix them 
thoroughly together and the surface will become hard 
when it dries out, and make a good road surface. It is 
only necessary to keep the surface smooth while it 
is drying. 

As a general thing, unless a sand road can be resur- 
faced, it is better to avoid disturbing it, and the less 
work done upon it the better. If there is but little 
travel over it, sod and weeds may grow in the sand, 
and these should not be eradicated. 



IMPROVEMENT OF COUNTRY ROADS. 93 

Art. 28. Gravel Roads. 

In the improvement of a country road, where the 
construction of a good Telford or macadam road can- 
not be undertaken, a surface of gravel may frequently 
be used to advantage, giving much better results than 
could be obtained with the surface of earth. Even 
a light layer of gravel may frequently prove of very 
great benefit. 

^^^here the subsoil is of a porous nature and well 
drained, a la^^er of three or four inches of gravel, or 
sometimes even less, well compacted, will constitute 
a very considerable improvement, especially if, as is 
usual with these light soils, the nature of the mate- 
rial of the road-bed is particularly unsuitable for the 
wearing-surface, dilHcult to compact sufficiently to 
shed water, and likelj^ to become soft when wet. 

Gravel for use on roads should be of hard, tough 
material, capable of resisting the abrasion of traffic. 
Natural gravels may differ widely in the character of 
the materials composing them, and in many instances 
are harder and more durable than the native stone of 
the same localit3\ Nearly any gravel will be an 
improvement upon an ordinar^^ road surface, but 
where an important road is being improved the material 
should be carefully selected. The size of pebbles 
composing the gravel is important in considering its 
value for road purposes. As a general thing they 
should not be more than I inch, or at most i^ inches, 
in greatest dimension. The size should not be too 
uniform, but the gravel should contain enough small 
fragments to fill the interstices between the larger 
pieces, in order that it may pack well in the road. 
When the gravel is too fine or too uniform^ it will not 



94 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

bond properly, and will be difficult to compact into a 
hard surface. The proper gradation of sizes is the 
most important characteristic of good gravel. The 
larger pieces are usually the hardest and most durable 
part of the gravel. They have resisted the grinding 
action which has reduced the other material to smaller 
fragments. It is desirable therefore that there be 
only enough fine material to fill the interstices in that 
of larger size. When fine material is in considerable 
excess the gravel should be screened in order to get the 
best results. In many instances, it is possible to 
greatly improve the quality of gravel by screening 
into two or three sizes and then recombining these in 
proper proportions to produce the most dense material. 

Binder. In order to bind well in the road surface, 
the small spaces between the fragments of gravel must 
be filled with fine material. Without this the frag- 
ments composing the gravel will roll upon each other 
and not pack well. Natural gravel may contain 
enough fine material or soft material w^hich will crush 
under the loads coming upon it to cause it to bind 
well in the road; or it may be necessary to add some 
material to the gravel surface to act as a binder. 
Clay, loam, or stone screenings may be used as a binder. 
It is desirable to use as little binder as is consistent 
with the proper bonding of the gravel. When in excess 
it has a tendency to cause the road to soften in wet 
weather and to crack in dry weather. This is espe- 
cially noticeable with clay binder. If gravel contains 
too much fine material, or when the fine material is 
unevenly distributed through the gravel, it should be 
passed over a one half inch screen, and the fine part 
thus removed be used on the surface as a binder. 

When gravel contains considerable large material. 



IMPROVEMENT OF COUNTRY ROADS. 95 

a screen of i ^ to i ^ inches mesh may be used to remove 
such material from the portion of the gravel to be used 
in the surface la\^er of the road. If the road is to be 
sufficienth^ thick to be constructed in two la3^ers, 
the larger pebbles screened from the gravel will be 
suitable for use in the lower course. 

Construction. In the construction of a road with 
gravel surface the road-bed should first be brought to 
the proper grade, with a form of cross section the same 
as that to be given the finished road. The gravel is 
then placed upon it and rolled to a surface, or left to 
be compacted b\' the traffic. It is alwaj^s advantageous 
when possible to compact the road b}^ rolling. The 
road-bed should be well rolled before placing the 
gravel, and the gravel surface afterward. A smooth 
hard surface may thus be produced, upon which the 
wheels of loaded vehicles ma}^ roll without producing 
any visible impression. 

In preparing a road-bed for gravel surface, w^hen a 
light coating of gravel is to be used, the surface of the 
ground is shaped up with the grader in the ordinary 
manner, but using a section flatter than the finished 
road is to have. The gravel is then placed and spread 
so as to have the proper thickness at the middle and 
diminish the thin edges at the sides of the road. On 
a good well drained road-bed, this construction with 
gravel four or five inches thick at the middle of the 
road Yna.y make a very good countr3" road. Good 
drainage is, however, essential to success with such a 
road. 

On important roads gravel is often used instead of 
broken stone, in the manner described in Chapter V. 
In many localities, gravel exists which is superior in 
hardness and durability to the local stone available 



96 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

for road metal. In such instances gravel is often 
used for surfacing a stone road. 

Maintenance. A new gravel road when first opened 
to travel may require considerable attention to keep 
it in good condition. The surface should be watched 
and all ruts or depressions which may form be at once 
filled up. When new material must be added in 
repairing a gravel surface, it should be fine and contain ' 
more binding material than the gravel used in the 
first construction of the road. 

After a gravel road is thoroughly compacted by the 
traffic, less attention is required to keep it in surface 
until it is worn thin enough to require resurfacing. 

Art. 29. Oiled Roads. 

The use of oil for hardening the surfaces of earth 
and gravel roads originated in California, where it 
has rapidl3^ extended into common practice. The use 
of oil was at first intended only for the purpose of 
laying the dust, and the surface of the road was 
sprinkled two or three times during the summer with 
a light coating of oil. The effect of the oil upon the 
road was such as to very quickly modify both the 
purpose and the method of the application, and many 
roads were soon constructed in which oil was used for 
the purpose of binding together the material of the 
road surface, and thus forming a crust over the road 
which would take the wear of traffic. The results in 
general were satisfactory, giving, in many instances, 
smooth firm road surfaces, free from dust during the 
summer, and without mud in winter. 

Methods of Construction. Several methods have been 
employed in the construction of oiled roads. In the 



IMPROVEMENT OF COUNTRY ROADS. 97 

earlier construction, the oil was applied to the road 
when hard and smooth, the surface being sprinkled 
with the oil, which was absorbed into the surface. 
The following extract from a paper by T. F. White, 
in Engineering Record for Feb. 22, 1902, explains this 
method as commonly practiced: 

''Oil on roads, besides aiding to make a wearing 
surface, preserves the road-bed. It follows therefore, 
that the road-bed should be carefully prepared, well 
graded and shaped, and the surface smoothed and 
packed as firmly as the material of which it is composed 
will permit before the oil is applied. We therefore do 
our grading during the earl}^ part of winter, that the 
road-bed may have the benefit of the winter rains, 
and become packed from travel as w^ell as from thor- 
ough rolling. We roll after it has become moistened 
through. Then in the spring, while still moist, we go 
over it with a blade grader or smoother or both, and 
dress up the surface, crowning it as desired; and as 
soon thereafter as the surface is dry and the weather 
is settled and warm, the oil is applied, as much in 
quantity as the material will absorb and mix w4th. 
This has reference to a road never oiled before. 

" It may be desired to put oil on a road that is not 
in very good shape as to grade and smoothness of 
surface. It is not recommended to apply oil to such 
a road, but circumstances may make it seem desirable. 
In such a road there may be chuck holes full of dust. 
To oil it we go over the holes first, scraping out the dust, 
filling them nearly full of oil, and then with hoe and 
rake, work in the dust, together with sharp sand and 
fine gravel, which are thrown in from a wagon drawn 
alongside, until the holes are filled from bottom up 
with oil, dirt, and gravel, thoroughly mixed together. 



98 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

When all the holes are filled, we apply a coat of oil to 
the whole surface of the road. Should the road be 
very uneven, however, and full of holes, we prefer to 
haul on gravel, of a kind that will pack, and fill up the 
holes and uneven places, saturate with water and roll 
before applying oil. Should the surface of a road be 
worked up to a considerable depth of dust, if it is of a 
nature to pack with water and compression, we drench 
it thoroughly and roll. But if it will not so pack, 
being of too sandy a nature, we pour on the oil, 
attempting to saturate all loose material to the firmer 
stratum below. The only rule we have as to quantity 
of oil to be applied is to put on sufficient to saturate 
all of the loose covering of the road, and secure some 
penetration into the firm road-bed beneath. 

"After the oil is put on in any of these instances 
some appliance for mixing the oil and loose road 
materials is run over the surface backwards and for- 
wards until a thorough mixing is accomplished. If 
the road surface is very loose, a common steel lever 
harrow, with the teeth slanted back, is useful. This 
may be dragged to and fro longitudinally along the 
road, and back and forth spirally across the road, 
until a thorough mixing is secured. On firmer roads 
and where there is little loose covering, a lighter 
implement, with numerous dragging fingers suspended 
from an axle, is better. 

"All this has reference mainly to roads that have 
never been oiled before. When it comes to oiling a 
road the second, third, and later seasons, the operation 
is somewhat different. Should the oiled surface be 
cut through and chuck holes formed (but there will 
be very few holes if the road has been properly looked 
after), we go over these in the manner previously 



IMPROVEMENT OF COUNTRY ROADS. 99 

noted for repairing chuck holes, and then apph^ a 
dressing of oil to the whole surface, just enough 
to saturate the loose material and secure a very slight 
penetration into the old oiled surface. Here I will 
call attention to a danger we may fall into, that of 
putting too much oil on the smooth, hard oiled surface 
we have previously obtained, softening it, and putting 
it in condition to rut, especially under heavy loads. 
We may in this w^ay lose a part of the results of the 
previous j^ear's work. I made this mistake on a road 
last summer, so can speak from experience. But 
enough oil should be put on to cover the entire surface 
as with a thin sheet. Then there will be a surplus of 
oil, and the road, if left without further attention, 
would be sticky and ver^^ unpleasant to travel over 
for a considerable time after the application. We 
therefore follow this application on smooth hard roads 
that have previously been oiled, with a sprinkling of 
sand, using fine gravel and sharp sand, such as builders 
use in their mortars. This takes up the surplus oil 
and adds to the wearing surface, and renders the road 
at once comfortable to travel over. The sand soon 
becomes incorporated with the rest of the road material 
and packs down smooth and hard. The quantit3" of 
sand put on is just sufficient to take up the surplus 
oil and no more. 

"We frequently use this sanding process also when 
applj^ing oil for the first time to a hard smooth road. 
We have used it on a macadamized road in which the 
surface was too tight to absorb the oil, and obtained 
excellent results. It is useful also where oil is applied 
to a tight adobe or other clay road. With the oil and 
sand a wearing surface may be built up on the cla3^ 
and be made to last, while without the sand the oil 



100 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

has a tendency to ball up with the clay dust and carry 
off. " 

Oil applied to a road surface in this manner is ab- 
sorbed by the material of the road covering to a small 
depth, varying, according to the character of the 
material, from about one half inch to li inches. This 
forms a thin coating of oiled material over the surface 
of the road, which prevents the formation of dust and 
assists in preventing water from penetrating into the 
road when rain comes upon it. 

In order to secure a greater penetration of oil, in 
some localities, the soil of the road is loosened by the 
use of a harrow to a depth of about two inches, before 
the application of the oil, and is then mixed thoroughly 
by harrowing again before being compacted by the 
roller. The method of application varies with the 
character of the soil, hard soil needing to be loosened 
and harrowed, or to have a coating of sand added, 
while sandy soils may be oiled without being disturbed. 

In oiling a road for the first time, two applications 
of oil are usually made, the second application being 
made at from one week to three months after the first 
one. In most instances the second application of oil 
is accompanied by a thin coating of sand or fine 
gravel, which takes up the surplus oil and forms the 
wearing surface of the road. 

Oil. The oil used for road improvement is commonly 
crude asphaltic petroleum, with specific gravity 1 1 
to 14 degrees, Baume, and containing from 30 to 60 
per cent of "D" grade asphalt. In the earlier work 
the oil was always applied hot, at temperatures from 
150 to 250 degrees Fahr. In later practice cold oil 
has frequently been used and each method has its 
advocates. 



IMPROVEMENT OF COUNTRY ROADS. lOI 

The amount of oil required varies with the charac- 
ter of the soil and the method of treatment. As 
much should be used as the soil will take up. The 
proper amount can onl}^ be judged b\^ experience w4th 
the soil to be treated. The quantit}^ of oil used varies 
from about one-half gallon to i^ gallons per square 
3^ard of road surface for the first application, and 
one-half to I gallon for the second application. The 
maintenance of the roads usuall}^ requires an applica- 
tion of oil each spring, the quantit3^ required decreas- 
ing from year to year. 

PETROLITHIC PAVEMENT. 

The ordinary oiled road consists of a light covering 
of oiled soil upon the road surface. In some instances 
oiled surfaces are made 4 to 6 inches deep, by loosen- 
ing the soil, saturating with oil, and then compacting 
b}^ rolling to a firm surface. Difficult}^ has been found 
in compacting so deep a laj^er of oiled earth, and this 
has led to the invention of the petrolithic rolling 
tamper for this purpose. The rolling tamper is shown 
in Fig. 20. It consists of a heavy roller with a large 
number of tampers, or feet, projecting from its surface. 
These feet pack the material, beginning at the bottom, 
and thus compress the whole layer to uniform density. 
An oiled road constructed with this machine is called 
a petrolithic pavement. The following specifications 
used in Los Angeles for this class of streets illustrate 
the method of construction emploj^ed in such work: 

"After the street has been brought to the required 
grade and cross-section as above specified, the sur- 
face shall be rolled with a roller weighing not less than 
250 pounds to the inch width of tire until the surface 



102 A TEXT-BOOK ON ROADS AND PAVEMENTS 

is unyielding. Depressions made by the rolling shall 
be leveled up with good earth and again jrolled. Such 




Fig. 20, 

portions of the street as cannot be reached by the 
roller, and all places excavated below grade and 
refilled, and all pipe trenches and other places that 
cannot be properly compacted by the roller, shall be 
tamped solid, and in case of wet weather or soft or 
muddy ground, making use of the roller unsafe or 
impracticable, the rolling shall not be undertaken until 
the ground has become sufficiently dr}^ 

" The street shall then be tested for grade and cross- 
section, and no further work shall be done upon it 
until a certificate shall have been issued stating that 
it is acceptable in these respects. It shall then be 
plowed to a depth of not less than six inches and 
thoroughly pulverized by cultivating and harrowing. 



OILING. 



"Oil shall then be applied as follows: 

" The area to be oiled shall extend from curb to curb 



IMPROVEMENT OF COUNTRY ROADS. IO3 

where there are no gutters, and where there are gutters 
then from gutter to gutter, including all intersections 
of streets and allej's, and to the property line on both 
sides of said intersections. 

"The roadwa}^ shall be coated evenly with the oil 
at the rate of one gallon to the square yard of surface 
covered. It shall then be thoroughly cultivated to a 
depth of 4 inches until the oil is well mixed with 
the soil. A second application of oil, at the rate of 
one gallon to the square yard of surface covered, shall 
then be made and the area shall be again cultivated 
to a depth of 4 inches until the oil and soil are w^ll 
mixed. The street shall then be plowed 4 inches deep 
with a plow that thoroughly turns over the furrows. 

" A third application of oil, to the extent of one gallon 
per square yard of surface covered, shall then be made, 
and the entire surface shall be thoroughly cultivated 
to a depth of 6 inches, a portion of the cultivating 
being done along diagonal lines so as to thoroughly 
mix the surface. The road-bed shall then be tamped 
with the tamping roller until it is solid to within 3 
inches of the finished surface. It shall then be graded 
with a road grader until it substantiall3^ conforms to 
the official cross-section, and shall then be tamped with 
the tamping roller until the entire surface is uniformly 
hard, solid and free from undulations or other irregu- 
larities. 

" The completed surface of the street must conform 
substantially to the established grade and cross- 
section. Should it be low, it shall be broken up to 
a depth of at least 2 inches, fresh earth and oil supplied 
and the surface again rolled as before. 

" Should an excess of oil remain upon the surface 
after it has been thoroughly completed, such oily 



104 A TEXT-BOOK ON ROADS AND PAVEMENTS^ 

portion shall be plowed to a depth of at least 6 inches 
and retamped. 

" In the process of cultivating, the surface shall be 
gone over not less than twice after each of the first two 
applications of oil, and not less than three times after 
the third application, and in all cases until the oil and 
soil are thoroughly mixed. 

" The total quantity of oil to be applied on the street 
shall be not less than three (3) gallons net oil by 
measure for every square yard of surface covered. 

"At all stages of the work sufficient water shall be 
applied to secure the best results in the tamping, the 
amount of water to be used to be governed by the 
character of the soil, the intention being to make 
the soil just damp enough to pack solid. 

"Any portion of the street that cannot be reached 
by the roller shall be tamped solid by hand, under the 
direction of the Board of Public Works. 

" The contractor will be held responsible for all 
damage to curbs, gutters, or cross-walks that may be 
caused by him in the performance of the work. 



OILo 

"The oil used shall be crude petroleum and shall 
conform to the following requirements: 

'' (a) Specific Gravity: The oil, after being freed from 
water and sediment, shall be of not less than eleven 
and five-tenths (11.5) degrees, and not more than 
fourteen (14) degrees, Baume, gravit3^, at sixty (60) 
degrees F, 

" The specific gravity shall be determined by the use 
of 'The Westphal Specific Gravit}^ Balance,' in con- 
junction with the accepted scale hereinafter described 



IMPROVEMENT OF COUNTRY ROADS I05 

for addition and deduction below or above normal 
temperature, 

"(b) Temperature: All oil must be delivered at the 
point required for sprinkling at a temperature of not 
less than one hundred (100) degrees nor more than one 
hundred and ninety (190) degrees F. 

''(c) Measurement: In determining the quantity of 
oil delivered, the correction for expansion by heat shall 
be as follows: From the measured volume of all oil 
received at any temperature above 60° F., an amount 
equivalent to 0.4 of one per cent for every 10° F. shall 
be subtracted as the correction for expansion by heat. 
For the purpose of measuring oil a temperature of 
60° F. shall be deemed normal temperature. 

''(d) Volatility: The oil shall not contain more than 
eight (8) per cent of matter volatile when said oil is 
heated slowly to two hundred and twenty (220) degrees 
F. and maintained at that temperature during fifteen 
(15) minutes. 

"(e) Asphalt: The oil shall contain not less than 
sixty (60) per cent of asphalt, having at a temperature 
of seventy-seven (77) degrees F. a penetration of eighty 
(80) degrees. District of Columbia Standard. 

" The percentage of asphalt shall be determined, using 
oil treated as described in Section (d) in the following 
manner: A weighed amount of said treated oil shall be 
heated, in an evaporating oven, to a temperature of 
four hundred (400) degrees F. until it has reached the 
proper consistency, when the weight of the residue 
shall be determined and the per cent calculated. 

" (f) Water and Sediment: Deduction will be made 
for water and sediment in exact proportion to the 
percentage of such water and sediment found therein, 
and the oil shall not contain over two (2) per cent of 



I06 A TEXT-BOOK ON ROADS AND PAVEMENTS. , 

such water and sediment as determined by the gasoline 
test. 

''(g) Tank Wagons : All tank wagons used for deliver- 
ing the oil must first be submitted to the Department 
of Oil Inspection, which will gauge and stamp into the 
steel heads of said tanks the capacity in gallons of said 
tanks, and no figures of capacity will be accepted other 
than the official rating given by the Department of Oil 
Inspection. 

" (h) All oil to be used shall be tested by the Depart- 
ment of Oil Inspection. 

ROLLER. 

"The tamping roller to be used in the execution of 
the work herein specified shall consist of a roller the 
outer surface of which shall be studded with teeth not 
less than 7 inches long and having a surface area of not 
less than 4 square inches each, the roller itself to be of 
such a weight that the load upon each tooth shall be 
not less than 300 pounds." 

Results of the Use of Oil on Roads. As already stated, 
good results seem to have been obtained in California 
with the use of oil both for laying dust on roads and 
for improving the resistance to wear and to the pene- 
tration of water into the road surface. The results 
obtained depend upon the character of the work and 
the care used in construction. To secure good results 
these roads require careful maintenance. The forma- 
tion of chuck holes due to the action of water upon the 
road is a principal difficulty, and these require prompt 
repair. A lightly oiled road surface is worn away by 
travel and water during the rainy season, and must 
be annually renewed. On the whole the results are 



IMPROVEMENT OF COUNTRY ROADS. lO/ 

reported as satisfactory and the use of oil is largely 
extending. 

California has a dr}^ climate, which is very favorable 
to this kind of construction. The object of the road 
improvement is rather to get rid of the dust, and cause 
the surface of the roads to hold together during the 
dry season, than to guard against the softening of the 
roads in wet weather. Under these conditions the use 
of oil constitutes a ver^^ desirable method of road 
improvement; while the occurrence of the asphaltic 
oil, which maj^ be obtained at low cost, makes possible 
economical construction. 

In considering the advisabilit\^ of extending such 
methods to other parts of the country, the differences 
of climate and of the purpose of road improvement 
should be taken into account, as well as the character 
of available materials. Some method of dealing with 
dust, other than that of sprinkling with water, is 
annually growing more important, while the breaking 
up, or raveling, of road surfaces during dry weather 
is a serious difhcultj^ particularly where there is con- 
siderable automobile travel. The paraffine oils of the 
Eastern states maj^ act quite differently from the 
California asphaltic oils^ while the greater amount of 
rainfall and differences in temperature will probabl}^ 
make oiled construction much more difficult to 
maintain in other parts of the countr3^ than in 
California. 

For California the value of these materials has been 
fulh^ demonstrated, although experience is likely to 
modify the methods of construction used. For other 
localities, the value both of materials and methods 
can be determined only by experiment. 



I08 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Art. 30. Sand-Clay Roads. 

In some of the Southern states where other materials 
for surfacing roads are not available, a mixture of sand 
and clay in proper proportions has been found a desir- 
able material for this purpose. In some instances 
the mixing of sand and clay for road work has received 
considerable attention, and good results have been 
obtained. The relative amounts of sand and clay to 
be used depend upon the character of the materials 
and can only be determined by experiment in each case. 
Where good materials are available, a fairly hard 
surface, well adapted to light traffic, may be obtained.- 
It is claimed that these roads are less noisy and less 
dusty than macadam. They require the same main- 
tenance as an ordinary earth road, but form a harder 
surface than earth usually found in a natural state. 
Mr. William L. Spoon, of the United States Office of 
Public Roads, has made an investigation and report 
upon this method of construction. 

* "'The best sand-clay road is one in which the wear- 
ing surface is composed of grains of sand in contact in 
such a way that the voids or angular spaces between 
the grains are entirely filled with clay, which acts as a 
binder. Any excess of clay above the amount necessary 
to fill the voids in the sand is detrimental. If a small 
section taken from the surface of any well constructed 
sand-clay road is examined with a magnifjang glass, 
the condition of contact which exists between the 
grains of sand and the small proportion of cla^^ which 
is required to fill the voids may be seen. Wherever 
this proper condition of contact exists for a few inches 

* U. S. Department of Agriculture, Oflke of Public Roads, Bulletin 
No. 27. 



IMPROVEMENT OF COUNTRY ROADS. lOQ 

in thickness upon the surface of the road, it will bear 
comparatively heavy traffic for a long time, even when 
the subsoil is sand or cla^^ 

'' All the experiments which have been made by this 
Office indicate that the materials should not be mixed 
in a dry state, but that they should be thoroughly 
mixed and puddled with water. It makes little 
difference by what method the stirring or mixing is 
done, so long as it is thorough and proper proportions 
of the materials are obtained. If an excess of cla\^ is 
used in the mixture, the grains of sand which are not in 
contact are free to move among and upon each other, so 
that no particle exerts more resistance to pressure than 
if the entire mass consisted of clay alone. On the other 
hand, if an insufficient amount of clay is used, the mix- 
ture will lack binding power and will soon disintegrate. 

" It has been pointed out that thorough stirring and 
puddling are absolutely essential to successful sand- 
clay construction. This is most easih^ brought about 
immediately after a hard or prolonged rain, the clay 
having been previousl}^ spread and the large lumps 
broken up as completely as possible. The surface 
should then be covered with a few inches of sand arid 
plowed and harrowed thoroughh^ by means of a turning 
plow and a cutaway or disk harrow. This stage of 
the work will of course be found somewhat disagree- 
able, leading, as it does, to the formation of a thick, 
pasty mud; but it is the onh^ practicable waj^ in which 
the necessary mixing can be accomplished. Many 
experiments have been tried with dry mixing of the 
claj^ and sand, but all have been more or less unsuccess- 
ful. In cases where the plowing and harrowing are 
considered too expensive, the mixing may be left to 
traffic. This, however, inevitably leads to a muddy 



no A TEXT-BOOK ON ROADS AND PAVEMENTS, 

road surface for a long time, although finally it is 
possible, by a proper distribution of the sand upon the 
clay, to bring about a fairly good result, even by this 
simple method/' 

"' It has already been shown that the best mixture for 
sand-clay construction is one in which there is just 
enough clay to fill the void in the sand, thus producing 
the proper cementing bond in the road surface. No 
exact rules can be laid down for calculating in advance 
the best mixture. It must be remembered that the 
relation of weight and volume will vary widely in 
different clays, according to the amount of water which 
they contain. Some clays, especially the more plastic 
varieties, even after they are as thoroughly dried as 
they can be by the hottest summer sun, will still hold 
as much as 20 per cent of water. This water is known 
to chemists as 'water of combination,' because it 
seems to be either combined with or held in the structure 
of the clay particles in such a way that it can only be 
driven out at a high temperature. It is apparent from 
this that in handling a clay of this kind, even when it 
seems quite dry, each ton will contain 400 pounds of 
water which does not enter into the consideration of 
volume. The amount of clay necessary to fill the voids 
in any given sand will therefore be found to vary/' 

'^ Practical experience has shown that the tendency 
is to calculate too little rather than too much sand for 
given amounts of clay, and almost invariablj^ a second 
and even a third application of sand is necessary over 
and above the calculated amount. It often happens 
that clay will work up to the surface under the action of 
traffic, in which case an extra top dressing of sand 
should be added when required. '' 

Upon a clay subsoil "the foundation having been 



IIMPROVEMENT OF COUNTRY ROADS. Ill 

properly prepared, the surface should be plowed and 
harrowed to a depth of about 4 inches until it is pul- 
verized as completely as possible. It is then covered 
with 6 to 8 inches of clean angular sand. The sand 
should be spread so that the layer is thickest at the 
center of the road, following in general the same method 
as was outlined for spreading clay upon a sandy founda- 
tion. The first mixing by plow and harrow is now 
done while the materials are still in a comparatively 
dry state. It has been found that the clay founda- 
tion can be more evenlj^ disintegrated when in that 
condition. After this first mixing has been finished 
the road is finally puddled with a harrow after a rain. 
In case an excess of clay works to the surface and tends 
to make the mixture sticky, sand is applied until this 
trouble is overcome. 

" Upon the completion of the mixing and puddling, 
the road should be shaped while it is still soft enough 
to be properl}^ finished with a scraper and at the same 
time stiff enough to pack well under the roller or under 
the action of traffic. In case it is impossible to obtain 
the proper consistency of the surface material, it is 
better to shape the road when somewhat too wet than 
when it is too dry, even if it is necessary to stop traffic 
upon it for a few daj^s. The road should be opened to 
traffic as soon as practicable after completion, as this 
will be found to have a beneficial effect upon it/' 

Art. 31. Miscellaneous Roads. 

Corduroy Roads. In timbered country, where roads 
must pass over wet and mudd}^ places, corduroy roads 
are sometimes employed. They are built by laying 
logs side by side across the roadway. By taking 



112 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

sufficient care in construction to select the logs and 
even up the spaces between them with smaller pieces, 
a reasonably smooth road may be built. Roads of 
this kind are at best very rough affairs, and require a 
great deal of work to keep them in fair condition for 
travel. They are of value only as temporary roads 
across bad places where the cost of a better road would 
be too great. 

Plank Roads. Many of the old toll roads in some 
parts of the country were plank roads, and at one time 
they were quite common. They are now rarely con- 
structed. The road is built of planks 2 to 4 inches 
thick and 8 or 9 feet long, laid upon two rows of stringers 
about 5 feet apart. The ends of the planks are not in 
line but are stepped off to assist wagons in passing from 
the side upon them. Usually a single line was used and 
teams turned out upon the earth road at the side in 
passing; but sometimes a double track was provided 
for teams in each direction. When in good condition, 
roads of this kind may be very good for travel, but they 
very quickly get out of repair and are not economical. 

Shell roads. In some localities where oyster shells 
are plentiful, these are used in constructing roads. 
They make a road very similar to that built of a soft 
limestone. The road is constructed in the same 
manner as with gravel, the shells being readily com- 
pacted by the traffic, and binding well in the road. The 
material is too soft to resist the wear of heavy traffic, 
and grinds up rapidly under travel. For localities 
where traffic is not heavy and a harder road covering 
would be expensive, these roads have often been found 
satisfactory and economical. 

Burnt-Clay Roads. In certain districts in the 
Southern states, sedimentary clays very commonly 



IMPROVEMENT OF COUNTRY ROADS. II3 

occur, and other road materials are not available. It 
has been proposed to form a road surface by burning 
the clay, and experiments have been made by the 
United States Office of Public Roads which seem to 
indicate that in many instances this may prove an 
effective means of road improvement in such localities. 

* " After grading the road to an even width between 
ditches, it is plowed up as deeply as practicable. It 
will be found necessary to use four horses or mules, as 
the extremely heavj^ nature of the cla3^ makes the work 
of deep plowing difficult. After the plowing has been 
completed, furrows are dug across the road from ditch 
to ditch, extending through and beyond the width to 
be burned. If it is intended to burn 12 feet of roadw^ay, 
the transverse furrows should be 16 feet long, so as to 
extend 2 feet on each side beyond the width of the final 
roadway. Across the ridges formed bj^ these furrows — 
which should be about 4 feet apart — the first course 
of cord wood is laid longitudinally so as to form a 
series of flues in which the firing is started. From 15 
to 20 of these flues are fired at one time. 

"The best and soundest cord wood is selected for 
this course and should be laid so that the pieces will 
touch, thus forming a floor. Another layer of wood is 
thrown irregularly across this floor, in crib formation, 
with spaces left between in which the lumps of clay 
are piled. Care should be taken that the clay placed 
on this cribbed floor is in lumps coarse enough to aUow 
a draft for easy combustion. 

" After the lumps of clay have been heaped upon this 
floor, another course of w^ood is laid parallel to the 
first. The third laj- er is laid in exactly the same manner 

* U. S. Department of Agriculture, Ofl5ce of Public Roads, Bulletin 
No. 270 



114 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

as the first, and each opening and crack should be 
filled with brush, chips, bark, small sticks, or any other 
combustible material. The top layer of clay is placed 
over all and the finer portions of the material are heaped 
over the whole structure. A careful arrangement of 
this cord wood cribbing to separate the clay is impor- 
tant, and the directions should be carefully followed. 

"The deep covering of clay which is thrown over all 
should be taken from the side ditches, and may be in 
lumps of all sizes, including the very finest material. 
It is spread as evenly as possible over the top in a 
layer of not less than 6 to 8 inches. Finally the 
whole is tamped and rounded off so that the heat will 
be held within the flues as long as possible. When 
coal slack is available the two top layers of wood 
may be omitted and the coal slack thoroughly mixed 
with the clay. 

" It is necessary to get the fires in the flues well 
under way before the first layer of wood is burned 
through. The first action of the fire is to drive out 
the water contained in the clay before the actual burn- 
ing and clinkering can begin. In burning the gumbo 
clays a great advantage is gained from the organic and 
vegetable matter which is contained in the clay, as 
that in itself aids combustion. " 

"After the firing is completed not only the portion 
of the clay which forms the top of the kiln but the 
ridges between the flues should be burned thorough^, 
so as to form a covering of burnt clay lo to 12 inches 
in depth, which, when rolled down and compacted, 
forms a road surface of from 6 to 8 inches in thickness. 
If properly burned, the material should be entirely 
changed in character, and when it is wet it should have 
no tendency to form mud. 



IMPROVEMENT OF COUNTRY ROADS. II5 

"When the material is sufficiently cooled the road- 
bed should be brought to a high crown before rolling, 
in order to allow for the compacting of the material. 
This can best be done with a road grader. After this 
the rolling should be begun and continued until the 
road-bed is smooth and hard. The finished crown 
should have a slope of at least one-half inch to the 
foot. " 

Slag Roads, Blast furnace slag is used in some 
localities as a material for surfacing roads. In some 
instances also slack from coal mines is used in the same 
way. Where these materials are available, they may 
provide a cheap method of improving the surfaces of 
roads of light traffic. Usually these materials are 
rapidly reduced to powder under any considerable 
traffic; in some instances, however, slag may be ob- 
tained which is hard and tough and forms a desirable 
road metal. 

Art, 32. Width of Tires, 

The effect of the width of wheel tires upon the 
resistance to traction has alreadj^ been mentioned in 
Art. 2. For ordinary roads, not in soft condition, 
tractive resistance is somewhat less for wide than for 
narrow tires. This difference, while not usually very 
great, is sufficient to be quite appreciable in the work 
of hauling heavy loads upon the roads. Narrow tires 
have a much more destructive effect upon a road 
surface than w^ide ones, and from the point of view 
of road maintenance, wide tires are very desirable. 
The concentration of a heav}^ load upon narrow wheel 
tires affords ver^^ little surface of contact between 
the wheel and road, and causes the wheel to indent the 
road surface, giving a powerful cutting action. The 



Il6 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

same load on a tire of sufficient width would tend to 
compact the road, acting like a roller, and if these 
wheels are so placed as not to run in the same track, 
the difference would be still more marked. There is, 
however, no advantage in an excessive width of tire. 
When this exceeds 4 or 5 inches, upon a properly 
crowned road, the tire will be only partially in contact 
with the road and the load will be carried on one edge 
of the tire, which will indent the road surface. 

The general introduction of wide tires upon vehicles 
traveling our highways would greatly simplify the 
problem of road maintenance, particularly upon earth 
roads. This fact is generally admitted and appre- 
ciated by road builders, but the practical difficulties 
met in attempting to change the prevailing system 
of narrow tires has been too great, and the agitation 
for wide tires has not as yet produced much effect. 
Many propositions have been made looking toward 
the regulation of the width of tires by law. This has 
not met with much success. In some states the laws 
provide for a rebate upon road taxes to persons using 
wide tires upon wagon wheels used for highway 
transportation. 

The usual w4dth of tire upon ordinary wagons is 
i^ or I J inches. For the best effect upon the high- 
ways, these should be increased so as to vary from 
about 3 to 5 or 6 inches, according to the load for 
which the wagon is designed. 

The wide tire is at a disadvantage on a distinctly 
bad road, and efforts to secure the adoption of wider 
tires can hardly meet with much success until very 
great improvement has taken place in the character 
of the country roads. Wider tires should naturally 
follow better roads and assist in maintaining them. 



CHAPTER V, 

BROKEN-STONE ROADS. 

Art. S3' Defixitiox. 

Brokex-stoxe roads consist essentially of a mass of 
angular fragments of rock deposited, iisualh^ in la\'ers, 
upon the road-bed or a foundation prepared for it, and 
then consolidated to a smooth and uniform surface b\^ 
means of a roller or by the action of the traffic which 
passes over it. 

There are two commonh^ recognized systems of con- 
structing broken-stone roads, differing in the nature of 
the foundation emploj^ed, and known respectively b\^ 
the names of the men who first introduced them into 
English practice as Telford roads and Alacadam roads. 

Each of these systems has been greath^ modified in 
use since the time of its founder, and each name is now 
used to cover a general class of constructions differing 
very materially within itself as applied in the practice 
of different engineers. Each of the systems also has 
its earnest adA^ocates, who contend for its exclusive use, 
and numerous controversies have been the result, at 
the conclusion of which each part}^ is "of the same 
opinion still. '' The view taken bN" different road- 
builders in this matter, it ma}' be remarked, appears to 
be the result usually of the local necessities of the 
vicinities in which thej- work, and of the skill with which 
the different S3'stems have been applied in work w^^/'ch 
has come under their observations. In road- building, 

117 



Il8 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

as in any other class of engineering works, no rigid 
rules can be laid down for universal application; each 
road must be designed for the place it is to occupj^ and 
the work it is to do. 

In some parts of this country natural gravel is sub- 
stituted for broken stone in the construction of these 
roads, the methods of construction being the same as 
in using broken stone. 

Art. 34. Macadam Roads. 

Macadam roads as commonly constructed consist of 
two or more layers of broken stone, each layer being 
rolled to a firm bearing before placing the next. The 
broken stone is usually placed directly upon the earth 
road-bed. 

In constructing a macadamized roadway, the road- 
bed is first brought to the proper grade in the usual 
manner, and rolled to a uniform surface. The surface 
of the road-bed is either fiat or raised at the middle to 
the same section as is to be given the finished road- 
surface. The inclined form is usually emplo3^ed, and 
seems preferable on account of affording better drain- 
age in case any water finds its way through the surface 
layer. 

On village streets where curb and sidewalks are 
employed, this section of the road-bed may extend to 
the curbing (as shown in Fig. 3) , but on country roads a 
bench of earth should be left at the side between the 
broken stone and the gutter in order to confine the 
broken stone while it is being compacted, and prevent 
the spread of the surface materials. The form of the 
road-bed before placing the stone would then be as 
shown in Fig. 21, where the completed road is to be of 



BROKEN-STONE ROADS. 1 19 

the form given in Figs. 5 and 17. Where the road-bed 
is in embankment, it is common to construct the earth 
embankment to the height of the finished surface, and 
afterwards excavate the material necessary to admit of 
placing the surface layers. The embankment should 
be allowed to settle and become thoroughly compacted 
before the broken stone is placed upon it, and it is 
desirable with new embankments that they be used for 
a short time by the traffic upon the earth surface be- 

Fig. 21. 

fore finishing the road; where, however, the material 
is well compacted in construction and can be thor- 
oughly rolled this is not necessarj^. 

In constructing the road-bed its proper drainage 
must be considered, and where necessary to prevent its 
becoming wet under the broken stone some means 
should be adopted to artificially drain it. 

Upon the completion of the road-bed, a layer of 
broken stone, usually from 3 to 5 inches in thickness, is 
placed upon it and thoroughh^ rolled. Upon this a 
second layer is placed and likewise rolled to a uniform 
surface. Sometimes a third laj^er is added, or in case 
of a very thin road it may consist of a single layer, the 
number of la3^ers depending upon the thickness of the 
road. When no roller is used, the stone is usually 
spread on the surface of the road-bed to the full thick- 
ness desired for the road, and left to the action of the 
traffic. 

The upper layer constitutes the wearing surface of 
the road, and upon this it is usually necessar^^ to place 



120 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

a thin layer of finer material called binding material, 
which may consist of rock chips, sand, small gravel, or 
sometimes loam, and is washed and rolled into the inter- 
stices of the rock, with the object of forming a com- 
pact and impervious surface. Binding material is in 
like manner often added to the lower layers of the 
road, although this has not been common practice. 
The object should be to fill the voids in the rock as 
completely as possible, serving to make the road one 
solid mass, to bind the rock more firmly together, and 
to prevent the percolation of water through the surface. 

Art. 35. Telford Foundations. 

The distinguishing feature of a telford road is its 
paved foundation. It consists essentially of a pave- 
ment of stone blocks set upon the road-bed and cov- 
ered with one or more layers of broken stone. 

In forming a telford road the road-bed is con- 
structed in the same manner as for macadam, being 
made either level or crowned. A pavement is then 
placed upon the road-bed from 5 to 8 inches thick, de- 
pending upon the thickness to be given the road 
material, the general practice being to make the pave- 
ment about two thirds of the total thickness of the 
road. The stones used for the pavement may vary 
from 2 to 4 inches in thickness and 8 to 12 inches in 
length; they are set upon their widest edges and with 
their greatest lengths across the road. The irregulari- 
ties of the upper part of the pavement are then broken 
off with a hammer, and all the interstices filled with 
stone chips and wedged with a light hammer so as 
to form a completed pavement of about the thickness 
required. 



BROKEN-STONE ROADS. 121 

Upon this pavement the layers of broken stone are 
placed, and the road-surface completed in the same 
manner as for a macadam road. 

The practice of Telford was to grade the road-bed 
flat, and then construct his pavement deeper in the 
middle than at the sides, using for a roadway i6 feet 
wide stones about 8 inches deep at the middle and 
5 inches at the sides. This practice is still followed by 
some engineers, but it is now more common and usually 
considered preferable to make the surface of the road- 
bed parallel to the finished surface and the pavement 
of uniform thickness. Fig. 22 shows a section of tel- 
ford road as now commonlv constructed. 




Fig. 22. 

The following extract from the specifications of Mr. 
James Owen, for telford roads in Essex County, New 
Jersey, may be regarded as representing the best prac- 
tice in such construction. 

"After the road-bed has been formed and rolled, as 
above specified, and has passed the inspection of the 
Engineer and Supervisor, a bottom course of stone, 

of an average depth of inches, is to be set by 

hand as a close, firm pavement, the stones to be placed 
on their broadest edges lengthwise across the road in 
such manner as to break joints as much as possible, 
the breadth of the upper edge not to exceed four (4) 
inches. The interstices are then to be filled with 
stone chips, firmly wedged by hand with a hammer, 
and projecting points broken off. No stone of greater 



122 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

length than ten (lo) inches or width of four (4) inches 
shall be used, except each alternate stone on outer 
edge, which shall be double the length of the others 
and well tied into the bed of the road; all stones with 
a flat smooth surface must be broken, the whole sur- 
face of this pavement to be subject to a thorough 
settling or ramming with heavy sledge hammers, and 

thoroughly rolled with a ton roller. No stone 

larger than one and one-half (i^) inches to be left 
loose on top of telford. ' 

The proper foundation to be used for a broken-stone 
road depends upon the nature and condition of the 
road-bed upon which it is to be constructed and the 
nature of the traffic to pass over it. If a firm, well- 
compacted, and thoroughly drained road-bed may be 
obtained, of material which will not readily soften 
under the action of moisture, there will usually be no 
need for a special foundation, but the first layer of the 
macadam may be placed directly upon the surface of 
the road-bed. If, however, the road-bed is of a ma- 
terial retentive of moisture, not thoroughly drained, 
and likely to become soft in wet weather, and the 
broken stone be laid immediately in contact with it, 
the stones of the lower layer of macadam may be grad- 
ually worked down by the weight of the traffic into the 
soft earth, and the soil at the same time work up into 
the voids in the stone, causing a gradual disintegration 
of the road. It may thus also become retentive of 
moisture and subject to the disrupting action of frost. 
In this case some foundation must be provided which 
is capable of resisting the penetrating action of the soft 
material of the road-bed and of distributing the load 
over it. 

It is not intended in the above to imply that the 



BROKEN-STONE ROADS. 1 23 

use of a foundation of this character should take the 
place of proper drainage. The advisability of artificial 
drainage should always be carefull}^ considered, and 
where the road is threatened by water which may be 
removed by the construction of drains they should be 
used, but frequently thorough drainage is difficult or 
doubtful, and it is desirable to adopt heavy construc- 
tion such as the telford foundation gives. 

It is commonl3^ claimed by the advocates of the 
macadam system of construction that on any well- 
drained and well-compacted road-bed there will be no 
tendenc}^ on the part of the stone to work down or of 
the soil to work up, and hence that the Telford foun- 
dation is an unnecessary expense. The difficulty of 
procuring a perfectl}^ stable and reliable road-bed in 
many localities is, however, very generally recognized, 
and telford pavements are largely used. 

The Massachusetts Highway Cominission has dis- 
continued the use of telford construction. In their 
report for 1903 the}^ give the following as their reasons 
for this course: 

"No telford foundations have been laid for two 
years past. Much of this class of construction has 
been done by the commission, and every contingency 
was supposed to have been carefully considered. 
Notwithstanding the careful attention to details, the 
results from the use of telford have been far from 
satisfactory. In a few cases the large stones have 
come to the surface in a manner w^hich would seem 
to indicate a movement due to frost action. In other 
cases, where a fairly soft native stone was used for 
surfacing, the upper courses were worn away so as 
to leave the large stones exposed. There are few, 
if any, cases where equally good results cannot be 



124 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

obtained by the use of sand, gravel, or small stones 
in place of telford, and at a less cost/' 

These difficulties do not seem to have been met by 
other road builders, and where conditions are such as 
to make advisable the construction of thick, heavy 
roads, telford construction is very commonly adopted 
as the cheapest form for such work. 

The Massachusetts Highway Commission has also 
adopted for difficult construction on wet, heavy soils, 
a blind center drain, and consider this cheaper than 
the telford construction. This is described in their 
report for 1904 as follows: 

"On heavy, wet soils a center 'V '-shaped drain 
has been substituted for the side drains and telfording. 
In building this type of road the earth is loosened and 
thrown out toward the sides, so as to give a 'V- 
shaped trench, with its greatest depth in the center 
of the proposed roadway. Narrow trenches are cut 
through the sides of this center trench, at intervals 
of 50 or more feet, connecting its lowest part with the 
gutters on the side, and placed at a depth and slope to 
thoroughly remove all water. The center and cross 
ditches are filled with field or wall stone, the depth 
of this stone varying from 12 to 18 inches at the 
center, and from 6 to 12 inches on the sides, the thick- 
ness being dependent upon the character of the soil 
in the sub-grade. The tops of these large stones are 
given a crown to receive the surfacing material.'' 

Art. 36. Rocks for Road Building. 

Properties Required. The surface material for broken- 
stone roads must bind together into a solid surface 
capable of bearing the loads which come upon it and 
of resisting the wear of the traffic. 



BROKEN-STONE ROADS. 12$ 

A stone to be durable in the surface of a road should 
be as hard and tough as possible. The qualities of 
toughness and resistance to abrasion are of more 
importance than hardness and resistance to crushing. 
A stone ma\^ be hard and brittle and quickly pound 
to pieces in a road surface, or it may have a high 
crushing strength and grind away quickly under 
abrasion, as is the case with some varieties of sand- 
stone. If, however, it be too soft, it may crush 
under the loads coming upon it, and thus lack in 
durability. 

A stone for a road-surface must also resist well the 
disintegrating influences of the atmosphere. It should 
be as little absorptive of moisture as possible in order 
that it may not be liable to injury from the action 
of frost. Many limestones are objectionable on this 
account. 

The material of a road-surface should also be mii- 
form in quality; otherwise the wear of the surface will 
not be even, and depressions will appear where the 
softer material has been placed. 

As the under parts of the road are not subject to 
the wear of the traffic, and have only the weight of the 
loads to sustain, it is evidenth^ not important that the 
foundation or lower layers be of so hard or tough a 
material as the surface; and hence it is frequently pos- 
sible, by using an inferior stone for that portion of the 
work, to greatly reduce the cost of construction. 

The binding of the road-surface into a compact mass 
capable of resisting the wear of traffic depends largely 
upon the cementing properties of the material. By 
the cementing power of the stone is meant that 
property which enables the fine dust to act, when wet, 
as a cement and bind the fragments of rock composing 



126 A TEXT-BOOK ON ROADS AND PAVEMENTS, 

the road-surface firmly together. This is perhaps the 
most important quality of the material, and a high 
cementing value is always desirable. The tenacity 
with which the fragments of rock are held together is 
perhaps more important in the wear of the road than 
the resistance to wear of the fragments themselves. 
The powdering of the cementing material in dry weather 
sometimes causes the loosening of the stones and the 
raveling of the broken-stone surface. This is more 
apt to occur where the road metal is hard and resistant 
to wear than where it grinds up more rapidly. 

The character of the material which will give the 
best results in a road-surface depends upon the local 
conditions under which the road is to be built and 
the traffic to which it is to be subjected. Under heavy 
traffic, hard and tough road metal is necessary to 
good results. Under lighter traffic, a softer rock may 
sometimes be better if it is coupled with good binding 
properties. 

" " Experience shows that a rock possessing all 
three of the properties mentioned in a high degree 
does not under all conditions make a good road material; 
on the contrary under certain conditions it may be 
altogether unsuitable. As an illustration of this, if 
a country road or city parkway, where only a light 
traffic prevails, were built of a very hard and tough 
rock with a high cementing value, neither the best 
nor, if a softer rock were available, the cheapest results 
would be obtained. Such a rock would so effectively 
resist the wear of a light traffic that the amount of 
fine dust worn off would be carried away by wind and 
rain faster than it would be supplied b^^ wear. Conse- 
quently the binder supplied by wear would be insuffi- 

* Engineering Record, May 17, 1902, 



BROKEN-STONE ROADS. 12/ 

cient, and if not supplied from some other source the 
road would soon go to pieces. The first cost of such 
a rock would in most instances be greater than that of 
a softer one, and the necessary repairs resulting from 
its use would also be very expensive/' 

The selection of material for road metal is commonh^ 
determined rather by the cheapness and convenience 
of location than by its desirability for the purpose. In 
most instances this is of necessity the case, and the 
availability of material in vicinitj^ of the work makes 
possible the construction of the road. It is, however, 
frequently possible, by judicious selection of materials, 
to greatly improve the results obtained in such work, 
and while the selection of a stone for road construction 
will of course alwaj^s depend largel3^ upon \vhat is to 
be obtained in the locality of the work, the importance 
of a thoroughly good material in the road surface is 
so great in its effect upon the durability and cost of 
repairs of the road that it may frequently be found 
economical, on roads subjected to a considerable traffic, 
to bring a good material a considerable distance rather 
than to use an inferior one from the immediate vicinit}^. 
It may also be suggested in this connection that in 
many instances railway transportation over a consid- 
erable distance may be small compared with wagon 
transportation over a short distance, and the impor- 
tation of good material may add but slightly to the 
aggregate cost of the work. 

Classification. The rocks used for road-building 
differ wddely in their mineral characters. The classifi- 
cation shown in the following table is proposed by 
Mr. Edwin C. E. Lord.- 

* U. S. Department of Agriculture, Office of Public Roads, 
Bulletin No. 31. 1907. 



128 A TEXT-BOOK ON ROADS AND PAVEMENTS. 



GENERAL CLASSIFICATION OF ROCKS. 



I. Igneous. 



II. Sedimentary, 



Intrusive (plutonic) ' 



2. Extrusive (volcanic) 



I. Calcareous. 



Siliceous : 



III. Metamorphic. 



a. Granite. 

b. Syenite. 

c. Diorite. 

d. Gabbro. 
^e. Peridotite. 

a. Rhyolite. 

b. Trachyte. 

c. Andesite. 

d. Basalt and diabase. 
Ta. Limestone, 
^b. Dolomite. 
[a. Shale. 
I b. Sandstone. 

Ic. Chert (flint). 

ia. Gneiss, 

b. Schist, 

c. Amphibolite. 

{a. Slate, 

b. Quartzite. 

c. Eclogite. 

d. Marble. 



" All rocks of the igneous class are presumed to have 
solidified from a molten state, either upon reaching the 
earth's surface or at varying depths below it. The 
physical conditions, such as heat and pressure, under 
which the molten rock magma consolidated, as well as 
its chemical composition and the presence of included 
vapors, are the chief features influencing the structure. 
Thus, we find the deep-seated, plutonic rocks coarsely 
crystalline with mineral constituents well defined, as 
in case of granite rocks, indicating a single, prolonged 
period of development, whereas the members of the 
extrusive, or volcanic, types, solidifying more rapidly 
at the surface, are either fine grained or frequently 
glassy and vesicular, or show porphyritic structure. 
This structure is produced by the development of large 
crystals in a more or less dense and fine-grained ground 
mass, and is caused generally by a recurrence of mineral 
growth during the effusive period of magnetic consoli- 



BROKEN-STONE ROADS. 1 29 

dation. Rocks of this kind, exhibiting a more or less 
spotted appearance, are commonly described as por- 
phyries, regardless of mineral composition, thus causing 
great confusion in the nomenclature. A movement 
in the rock magma while cooling causes frequently a 
banded arrangement of the minerals, or flow structure.'' 

"Igneous rocks varj^ in color from the light gray, 
pink, and brown of the acid granites, syenites, and 
their volcanic equivalents (rhj^olite, andesite, etc.) to 
the dark steel gray or black of the basic gabbro, 
peridotite, diabase, and basalt. The darker varieties 
are commonly called trap. This term is in very general 
use and is derived from trappa, Swedish for stair, 
because rocks of this kind on cooling frequently break 
into large tabular masses, rising one above the other 
like steps, as may be seen in the exposures of diabase 
on the west shore of the Hudson River from Jersey 
city to Haverstraw. 

" The sedimentary rocks as a class represent the con- 
solidated products of former rock disintegration, as in 
case of sandstone, conglomerate, shale, etc., or they 
have been formed from an accumulation of organic 
remains chiefly of a calcareous nature, as is true of 
limestone and dolomite. These fragment al or clastic 
materials have been transported by water and deposited 
mechanically in layers on sea or lake bottoms, producing 
a very characteristic bedded or stratified structure in 
man}^ of the resulting rocks.'' 

" Metamorphic rocks are such as have been produced 
b}^ the prolonged action of physical and chemical 
forces (heat, pressure, moisture, etc.) on both sedi- 
mentary and igneous rocks alike. The foliated t^^pes 
(gneiss, schist, etc.) represent an advanced stage of 
metamorphism on a large scale (regional metamor- 



I30 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

phism), and the peculiar schistose or foliated structure 
is due to the more or less parallel arrangement of 
their mineral components. The nonfoliated types 
(quartzite, marble, slate, etc.) have resulted from the 
alteration of sedimentary rocks without materially 
affecting the structure and chemical composition of 
the original material/' 

The rocks commonly used for road-building may be 
classified according to their popular designations as 
trap, granite, limestone, sandstone, and chert. 

Trap. The term "" trap " is commonly applied to most 
of the volcanic igneous rocks used for road-building, 
including basalt and diabase. While these rocks vary 
considerably in character, they are usually very com- 
pact and tough, and may be classed as the best material 
for roads of heavy traffic. 

* " It is characteristic of all the trappean rocks that 
they have once been fluid from heat and while in that 
state have been injected into fissures of the rocks 
through which they have found their way toward the 
present surface of the country. Only in rare cases 
have they actually passed upward to the surface of 
the earth toward which they moved; their motion was 
arrested in the lower levels of the rocks to which the 
surface has been brought down by the agents of atmos- 
pheric decay. The result of their consolidation under 
the conditions of pressure in which thej^ cooled has 
caused these originally molten materials to be very 
compact, a state which is favored also by their chemical 
composition. This causes the materials to be very 
solid and elastic. They generally resist decay in such a 
manner that they often project above the surface, while 
the softer rocks on either side have been worn down." 

* Shaler's American Highways, p. 56. 



BROKEN-STONE ROADS. I3I 

Granite. The granites, including syenite and gneiss, 
van^ widely in character and differ greatly in value as 
road materials. They may be classed as next in value 
to trap for wear in the road-surface, but are somewhat 
deficient in cementing properties. 

* " In an examination of the bearing of the petro- 
logical characters upon the attrition results in this 
group three prominent factors stand out. They are: 
(i) Texture, (2) the kind of mineral, (3) the state of 
freshness of the minerals. With regard to the first of 
these it is evident that fineness and evenness of grain 
is an advantage, and that coarse grain or porphyritic 
structure is disadvantageous. It is on account of the 
granitic texture that the rocks of this group, taken as 
a whole, are not higher in the attrition scale. 

"The influence of the Idnd of mineral (2) is not so 
easy to determine, but, other things being equal, a 
high proportion of hornblende appears to be favorable 
to resistance; quartz in a like manner is favorable 
because of its hardness and lack of cleavage. 

" Fresh unaltered original minerals are not absolutely 
essential to a high capacity to resist abrasion; the two 
stones that take the best position in the test scale for 
this group are considerably altered — the feldspars are 
decomposed, and their substance is a mixture of smaller 
mineral units; the ferro-magnesian minerals have 
changed to chlorite and to fibrous uralitic hornblende.'' 

I " In the case of the igneous rocks it will be noted 
that the plutonic types with granitic granular structure 
(granite, syenite, diorite, and gabbro) are, as a rule, 
harder but inferior in toughness to their volcanic 

* Lovegrove, Attrition Tests of Road-making Stones, p. 59. 

t U. S. Department of Agriculture, Office of Public Roads, Bulletin 
31- 



132 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

equivalents (rhyolite, basalt, and diabase). Tliis is 
due to the more fully crystalline condition and coarser 
grain of the plutonic rocks. In the case of the volcanic 
types a compact cr3^stal intergrowth and fine grain 
tend to increase the toughness rather than hardness of 
the material. The deleterious effect of atmospheric 
decomposition on rock texture is especially noticeable 
in the case of peridotite, andesite, and altered basalt, 
where the indifferent results of the physical tests, 
excepting cementing value, may be directly ascribed 
to the presence of such soft secondary minerals as 
kaolin, serpentine, calcite, chlorite, etc. ... As has 
already been stated in a previous paragraph, the 
cementing value is as a rule found more highly 
developed in the igneous rocks which contain alteration 
products than in their unaltered varieties. This is 
especially true in the case of diabase and basalt, rocks 
very similar in origin and mineral composition. Con- 
tinuing a step further, we note a marked decrease in 
toughness, hardness, and resistance to wear in the 
altered varieties of both these rock types over their 
fresher representatives. This is in line with what has 
already been said and indicates that the presence of 
secondary minerals in appreciable quantities, whether 
because of their softness or their indefinite semi- 
crystalline condition, weakens the original mineral 
bond and tends to destroy the primary texture of the 
rock, while at the same time furnishing the elements 
for a high binding quality in the rock powder. Valuable 
results bearing on the decomposition of rock powders 
by water have been obtained by Dr. A. S. Cushman in 
a series of interesting experiments carried on in the 
chemical laboratory of this Office. Doctor Cushman 
has shown that hydrolysis takes place in case of many 



BROKEN-STONE ROADS. 133 

rock powders the moment thej^ are wet, thus pro- 
ducing secondary products (hydrated silicates) of a 
colloidal nature which greatly increase the binding 
power. This points finally to the conclusion that the 
mineral analysis of igneous rocks, besides providing a 
convenient means for comparison and classification, 
serves to a certain extent as a measure of their phj^si- 
cal properties.'' 

Limestone. Limestones commonly possess the 
cementing power in fair degree, although lacking in 
hardness and resistance to abrasion. The cementing 
power has probably been commonly overestimated, 
because of the softness of the rock and the ease with 
which it usually packs in the road surface. Limestones 
are the most widely distributed and most generally 
used materials for road surfaces. They differ very 
widely in character, some forming an excellent material 
under moderate traffic and others being so soft as to 
offer little resistance to wear. 

* "In proportion as limestone becomes crystalline, 
i.e., takes on the character of marble, its value in road- 
making diminishes, for the reason that the crj^stalline 
structure in most cases so far weakens the mass that 
it is apt readily to pass into the state of powder. As 
these marbles occur only in districts where better road- 
making materials are likely to be present, they may not 
be further mentioned, except to say that their use is 
commendable for foundation la^'ers, where their fair 
cementation value makes them tolerably fit for service. 
So long as the bits are kept from the destructive action 
of the wheels and feet of the carriages and horses, they 
lend themselves to the road-master's use. Even where 
a more resisting top covering of ordinary broken stone 

* Shaler*s American Highways, p. 6i. 



134 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

cannot be provided, a tolerable road can be made of 
this material, often very cheaply by using the waste 
from quarries, by covering the surface with a coating 
of ferruginous matter, such as is afforded bj^ the leaner 
iron ores, or by using a top coating of gravel." 

* " If we take the pure dolomites alone, it is clear that 
those behave best in the attrition test which have 
a fine-grained, even, granular texture with irregular- 
shaped grains interlocking closely one with another, 
and with a general absence of porous cavities. A dolo- 
mite is by no means always better than a limestone, 
but the best type of dolomite will be more resistant 
than the best limestones, being harder. 

''Among the limestones, those stand highest that are 
composed of a mixed assemblage of small organic 
remains, notably of foraminifera, and possess at the 
same time a somewhat bituminous composition (this 
characteristic is often associated with foraminifera in 
carboniferous limestone). Crinoidal limestones do not 
stand so high, evidently on account of the ready 
cleavage of the particles of calcite, which is not only 
a soft mineral but has an extremely perfect cleavage, 
hence it wears rapidly and crumbles easily under 
repeated small blows; but if the crinoid fragments 
are small and uniform in size, set in a matrix of fine 
calcareous matter, the stone may compare well with 
other limestones. " 

Sandstone. As a class sandstones are deficient in 
cementing power and do not stand well in the surface 
of a road. They have commonly been sweepingly 
condemned and rejected by road-builders. In some 
instances, however, sandstones have given good 
results, and some of them possess fairly good cementing 

* Lovegrove, Attrition Tests of Road-making Stones, p. 6i. 



BROKEN-STONE ROADS. 135 

power. The value of a sandstone depends mainly 
upon the character of the cementing medium; where 
this is of siliceous character, a high degree of hardness 
and resistance to wear may result. 

''' "Considering the next important group of road- 
making rocks, we notice here also a marked coincidence 
in mineral composition and physical properties. The 
soft and nonresistant calcareous rocks (limestones, 
dolomites, and calcareous sandstones), composed largely 
of calcite and dolomite, are, as would be expected, 
inferior in hardness, toughness, and wearing qualities 
to the more siliceous sandstones and cherts." 

Chert. Chert is a very hard material and shows 
good resistance to w^ear. It is somewhat low in cement- 
ing value, but when carefully used forms a good road 
material. It is quite variable in character and needs 
careful selection. Chert is commonh^ found in a 
finely divided condition, and 'can be used, in many 
instances, without crushing. It occurs throughout 
many of the Southern states, where it is found widely 
distributed and is the only available material for such 
work. 

* " The low cementing value of chert obtained by 
laboratory tests is not in every case in accordance with 
that developed by this rock under traffic. In dis- 
cussing the origin of road material it has been stated 
that chert or flint belongs to that class of sedimentary 
rocks whose mineral components have been formed 
largely b\^ chemical precipitation and were originally 
of a colloidal or amorphous nature. The highly 
fractured condition of man3^ cherts is probably due in 
large measure to shrinkage caused by a decrease in 

* U. S. Department of Agriculture, OflQce of Public Roads, Bulletin 
31- 



136 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

volume in passing from an amorphous to a crystalline 
state. Although no experiments have as yet been made 
on the solubility of this material, it seems to the writer 
very probable that the dissolving action of road waters 
on finely divided chert dust is relatively high and that 
the high binding power of some of these rocks is 
caused by hydrated opaline silica resulting from a 
decomposition of this kind„ The fact that in certain 
localities surface flints are superior to quarry flints 
for road making is suggestive in this connection/' 

Mr. Lord, in the bulletin already quoted, gives tables 
showing the average mineral composition and plates 
showing structure of the various rocks, and indicates 
that the probable value of a rock for road-building 
may be inferred from its mineral composition and 
structure. 

■ ' To explain the bearing of mineral composition and 
structure on the physical properties of rocks, it has 
been found necessary to define these properties and 
describe the various methods for testing road materials. 
The results of these tests have been used in correlating 
the physical properties of the various rock families 
with their mineral components, and the following 
conclusions have been reached: 

""(i) Igneous and metamorphic rocks, owing to a 
high degree of crystallization and a preponderance of 
silicate minerals, offer a greater resistance to abrasion 
than nearly all varieties of sedimentary rocks. 

" (2) The coarse-grained intrusive rocks of the igneous 
class are harder, but break more readily under impact 
than the finer-grained volcanic varieties of like mineral 
composition. 

'' (3) The deleterious effect of atmospheric weathering 
on the wearing qualities of rocks has been demonstrated. 



BROKEN-STONE ROADS. 137 

"(4) The cementing value of rocks is, to a certain 
degree, measured b}^ the abundance of secondary 
minerals resulting from rock decaj^. 

'' (5) Metamorphic rocks have, as a rule, a low bind- 
ing power, owing to a regeneration of secondary 
minerals and to the effects of heat and pressure. The 
foliated types part readily along planes of schistosity 
and therefore are not well adapted to road con- 
struction. 

"(6) The quantitative mineral anah'sis of rocks 
serves to a certain extent as a measure of their useful 
properties for road construction. '" 

' Art. 37. Methods of Testing Stone. 

Final judgment concerning the value of stone for 
road purposes, or the best method of using it, can only 
be formed through experience with the material in use. 
Tests may, however, be applied which will throw much 
light upon the probable value of a material, or which 
may give an idea of the probable relative values of 
different available materials in a particular case. 
These tests are of two kinds: i. Determination of 
the mineral composition through petrographic analysis. 
2. Tests of the physical properties of road materials. 

PETROGRAPHIC ANALYSIS. 

The following methods of examination have been 
used by the Oflice of Public Roads of the U. S. Agri- 
cultural Department, and are described by Mr. Edwin 
C. E. Lord in Bulletin No. 31, August, 1907. 

"Upon receipt of the rock sample, which, accord- 
ing to the specification of this Office, should weigh not 



138 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

less than 30 pounds and be collected with care to repre- 
sent as nearly as possible an average of the whole 
exposure, it is examined in a general way to determine 
the proper method of analysis. 

" Rocks consisting essentially of the carbonates of 
lime and magnesia (limestones, dolomites, etc.), as 
well as fine-grained shales and unconsolidated sedi- 
mentary deposits, such as clays, sands, gravels, etc., 
are analyzed chemically when necessary, whereas all 
other materials are prepared for microscopic exami 
nation or determined macroscopically. 

"The mineral composition of a rock may, under 
favorable conditions, be estimated with considerable 
accuracy by a macroscopic examination, yet for exact 
quantitative results the aid of a polarizing microscope 
and transparent thin sections of the rock are essential 

Macroscopic Method. " The macroscopic form of 
analysis can be applied only to coarse-grained rock, in 
which the various mineral components are easily 
detected with the unaided eye. The approximate 
volumetric relations of these minerals may be deter- 
mined by preparing a smooth surface of the rock 
sample and covering it with a transparent celluloid 
scale divided into 100 equal square areas and estimat- 
ing the minerals present from the number of areas 
covered by each mineral. Any properh^ graduated 
scale can be used, but a transparent one is preferable. " 

Microscopic Methods. "Owing to the large amount 
of material received in this laboratory it has been 
found necessary to perfect a more rapid method of 
quantitative analysis than any hitherto described. 

"The laboratory is equipped with an exceptionally 
good petrographic microscope of the latest Fuess model, 
which, beside the usual attachments, is provided with 



BROKEN-STONE ROADS. 139 

a revolving anal^^zer in the tube to aid in the deter- 
mination of very low double refracting minerals, and 
a Schwarzmann scale for the measurement of optical 
axial angles. 

''Another important accessory is a detachable 
screw-micrometer, movable in the focal plane of the 
ocular b}^ means of a drum screw, w^hich, with" the 
most powerful objective (one-twelfth-inch oil immer- 
sion), records a drum-interval of 0.00004 mm. The 
measuring apparatus devised by Mr. L. W. Page and 
used for the mineral determinations consists of an 
ordinary fixed eyepiece having a square field divided 
into 100 quadratic areas. With the aid of this cross- 
line field, each square of which is one one-hundredth 
of the whole field, the relative proportions, expressed 
in per cent, of the minerals occupying the field can be 
readily determined by simply noting the number of 
squares covered by each mineral in turn. Averages 
derived from numerous examinations of this kind in 
various parts of the section indicate the percentage of 
the different minerals constituting the rock itself . '^ 

"Experience has shown that with a large majority 
of rock samples twent}^ determinations, using a magni- 
fication of 52 diameters, give very satisfactory results. 
In the case of extremely fine-grained rocks, however, 
it is best to use a three-quarter-inch objective lens 
which enlarges 105 diameters when combined with the 
eyepiece micrometer. 

"With rocks having an average grain exceeding 
5 mm., or those varying greatly in texture, as in the 
case of porphyritic and schistose varieties, it is in 
some instances well to emplo^^ a two-inch objective in 
combination with an ocular prepared in the same 
manner as that just described, but divided into only 



140 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

25 square areas and magnifying 30 diameters. In 
the case of these exceptionally coarse-grained rocks, 
two or more thin sections of the same sample are 
examined before reliable results can be obtained. " 



PHYSICAL TESTS. 

The physical properties of stone for road-building 
are commonly tested by determining the percentage 
of wear, using the Deval abrasion apparatus, and the 
cementation properties by the use of the Page cementa- 
tion test. Tests are also sometimes made of the crush- 
ing strength, resistance to impact, and resistance to 
abrasion by grinding, and, in some instances, the 
specific gravity and absorption of the rock are deter- 
mined. 

Abrasion Test. This test was first used in France, 
and is commonly known as the Deval test, bearing the 
name of its designer. The Deval machine consists of 
cylinders 20 cm. in diameter and 34 cm. in depth, 
closed at one end and with a tightly fitting cover for 
the other. Two or four of these cylinders are mounted 
upon a horizontal shaft so that the axis of each cylinder 
is inclined at an angle of 30 degrees with the axis of 
rotation. 

The method of conducting the test in the investiga- 
tions of the U. S. Office of Public Roads is as follows : * 
*'The sample to be tested is first broken in pieces that 
will pass in all positions through a 6 centimeter (2.4 
inch) ring. The stones are then cleansed, dried in a 
hot-air bath at 100 degrees C, and cooled in a desiccator. 
Five kilograms are weighed and placed in one of the 
cylinders, the cover bolted on, and the machine rotated 

*Bureau of Chemistry, Bulletin No. 79. 



BROKEN-STONE ROADS. I4I 

at the rate of 2000 revolutions per hour for 5 hours. 
When the 10,000 revolutions of the machine are com- 
pleted the contents of the cylinder are placed on a 
sieve of 0.16 centimeter {j\ inch) mesh, and the 
material which passes through is again sifted through 
a sieve of 0.025 centimeter (o.oi inch) mesh. Both 
sieves and the fragments of rock remaining on them 
are held under running water till all adhering dust is 
washed off. After the fragments have been dried at 
100 degrees C. and cooled in a desiccator they are 
w^eighed, and their weight subtracted from the original 
5 kilograms (ii pounds). The difference obtained 
is the weight of the detritus under 0.16 centimeter 
{^\ inch) worn off in the test. '' 

In the French experiments it was found that the 
best grades of rock gave about 20 grams of detritus 
per kilogram of rock tested, and the number 20 was 
adopted as a standard and the "coefficient of wear'' 
determined from the formula: 

^ rr • r 20 4OO 

Loemcient 01 wear = 20 X r— = — — - > 

W W 

in which W is the weight in grams of detritus under 
0.16 centimeter (^ inch) in size obtained per kilo- 
gram {2.2 pounds) of stone. The French coefficient 
is sometimes used in stating results in American tests, 
but it is more common to ues the "percentage of 
wear," w^hich is found by stating the weight of detritus 
under 0.16 centimeter in terms of percentage of the 
weight of rock tested. In this case. 

Percentage of wear 



Coefficient of wear 
In some of the work of the United States Agricul- 
tural Department another coefficient, known as the 



142 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

United States Agricultural Department coefficient of 
wear, has been employed. This coefficient is found by 
subtracting 4000 grams from the weight of the frag- 
ments over 3 centimeters (1.2 inches) which remain 
after the test and dividing the difference by 10. By 
this method, if 20 per cent of the material is abraded 
from the original 5000 grams, the coefficient is and 
the material considered worthless; if no dust is worn 
off, the coefficient is 1 00. 

The Committee on Standard Tests for Road Materials, 
of the American Society for Testing Materials, in 1904 
recommended the following specification for the 
abrasion test: ''The machine shall consist of one or 
more hollow iron cylinders, closed at one end and 
furnished with a tightly fitting iron cover for the 
other; the cylinders to be 20 centimeters in diameter 
and 34 centimeters in depth inside. These cylinders 
are to be mounted on a shaft at an angle of 30 degrees 
with the axis of rotation of the shaft. 

" At least 30 pounds of coarsely broken stone should 
be available for a test. The rock to be tested should 
be broken in pieces as nearly uniform as possible, and 
as nearly 50 pieces as possible shall constitute a test 
sample. The total weight of rock in a test should be 
within 10 grams of 5 kilograms. All test pieces should 
be washed and thoroughly dried before weighing; 
10,000 revolutions at the rate of between 30 and 2)d> to 
the minute, must constitute a test. Only the per- 
centage of material worn off which will pass through 
a 0.16 centimeter mesh sieve should be considered 
in determining the amount of wear. This may be 
expressed either in the per cent of the 5 kilograms 
used in the test, or the French coefficient, which is in 
more general use, may be given/' 



BROKEN-STONE ROADS. I43 

» 

^Cementation Test. This test was developed by 

Mr. Logan Waller Page while geologist of the Mas- 
sachusetts Highway Commission. It consists in grind- 
ing the stone into dust, wetting and moulding the 
dust into a small cjdinder, which is dried and then 
tested by subjecting it to the impact of the falling 
weight. The method of conducting this test as used 
b}^ the Office of Public Roads of the United States 
Department of Agriculture is as follows: "One 
kilogram of the rock to be tested is broken sufficiently 
small to pass a 6 millimeter but not a I millimeter 
screen. It is then placed in a ball mill and is ground 
for two hours and a half. This ball mill contains two 
chilled iron balls which weigh 25 pounds each, and is 
revolved at the rate of 2000 revolutions per hour. It 
was found b}' experiment that grinding rock thus pre- 
pared for two hours and a half was sufficient to reduce 
it to a powder that would pass through a 0.25 milli- 
meter mesh. The dust thus obtained is mixed with 
water to about the consistency of a stiff dough, and 
is kept in a closed jar for twenty-four hours. About 
25 grams of this dough is placed in a C3dindrical metal 
die 25 millimeters in diameter. A closel3^ fitting plug, 
supported b3'' guide rods, is inserted over the material, 
which is then subjected to a pressure of 1 00 kilograms 
per square centimeter. 

"It is most important that these briquettes should 
be compressed in a uniform manner, and for this a 
special machine has been designed. The die is placed 
on an iron platform supported by a piston rod, which 
is connected directly with a h^^draulic piston below. 
Water from a tank is admitted to the hydraulic cjdinder 
through a small orifice in the pipe. As the piston 
rises the platform and die are carried up with it, the 



144 A TEXT-BOOK ON ROADS AND PAVEMENTS, 

plug of the latter coming in contact with a yoke 
attached to a properly weighted lever arm. When the 
lever arm is raised one-eighth of an inch it closes an 
electric circuit which trips a right angle cock, shutting 
off the water and opening the exhaust. One minute is 
required to compress a briquette, and the maximum 
load is applied only for an instant. By this device 
practically uniform conditions are obtained. 

''The height of the briquette is measured, and if it 
is not exactly 25 millimeters the required amount of 
material is added or subtracted to make the next 
briquette the required height. Five briquettes are 
made from each test sample, and allowed to dry 
twelve hours in air and twelve hours in a steam bath. 
After cooling in a desiccator they are tested by impact 
in a machine especially designed for the purpose.'' 

The machine commonly used for this purpose is 
known as the Page- Johnson Impact Machine. It was 
designed by Mr. L. W. Page and afterward modified 
by Mr. A. N. Johnson. The blow is delivered by a 
hammer weighing one kilogram striking upon a 
flat -end plunger, which is pressed upon the briquette 
by two light spiral springs. The standard fall of the 
hammer for a test is I centimeter (0.39 inch), and 
this blow is repeated until the bond of cementation of 
the material is destroyed. The number of blows 
required is noted and the average obtained upon five 
briquettes is given as the cementing value. 

In making this test the results may be considerably 
affected by slight differences in manipulating the 
material. It is important that the same amount of 
kneading be used in all tests and that the dough should 
be allowed to stand at least 24 hours before forming 
the cylinders. 



BROKEN-STONE ROADS. I45 

Grinding Test. The test for abrasion by grinding 
is sometimes used in France, where it is known as the 
Dorr3^ test. It has also been used by the Office of 
Public Roads at Washington. The object of the test 
is to give a measure of the hardness of the rock. It 
gives interesting information concerning the material, 
but is not of special value in testing road materiaL 
The test is made as follows: "The test piece in the 
form of a cjdinder about 3 inches in length by i inch 
in diameter is prepared by an annular core drill and 
placed in the grinding machine in such a manner that 
the base of the cylinder rests on the upper surface of 
a circular grinding disk of cast iron, which is rotated 
in a horizontal plane by a crank movement. The 
specimen is weighted so as to exert a pressure of 
250 grams per square centimeter against the disk, 
which is fed from a funnel with sand of about li 
millimeters in diameter. After 1000 revolutions the 
loss in weight of the sample is determined and the 
coefficient of wear obtained by deducting one -third 
of this loss from 20." 

Impact Test. This test is intended as a measure of 
the toughness of the material. It is frequently made, 
although not of special value as a test for road material. 
It is made as follows: "The test piece is a cylindrical 
rock core similar to that used in determining hardness 
and the test is made with an impact machine con- 
structed on the principle of the pile driver. The blow 
is delivered by a hammer weighing 2 kilograms, w^hich 
is raised b^^ a sprocket chain and released automatically 
by a concentric electro-magnet. The test consists of a 
I centimeter fall of the hammer for the first blow and 
an increased fall of I centimeter for each succeeding 
blow until the failure of the test piece occurs. The 



146 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

number of blows required to cause this failure repre- 
sents the toughness. "" 

The abrasion and cementation tests are frequently 
employed for the purpose of comparing the properties 
of various road stones, and afford a means by which a 
judgment may be formed as to the probable relative 
values of various materials for road construction. No 
fixed standard for comparison has been devised, and the 
relative importance of the various properties depends 
upon the character of the road to be constructed. 

Art. 38. Road Metal. 

Stone is prepared for use in road work by crushing and 
screening. In the early days of broken-stone roads, 
all stone was broken by hand, and the roads were care- 
fully constructed of stone broken to approximately uni- 
form sizes without the addition of a binding material. 
The development of stone crushing machiner3^ has, 
however, modified practice in this regard and stone 
crushed by machinery is now almost exclusively used„ 
It gives satisfaction both as to binding properties and 
durability, and has the advantage of greatly lessening 
the cost. 

The size to which stone should be broken for road 
material depends to some extent upon the nature of 
the material. The harder and tougher it is the smaller 
the pieces may be without danger of crushing or shat- 
tering under the loads and shocks received in the road 
surface, and the smaller also thej^ will need to be in 
order to be thoroughly compacted in the road. 

There is a difference of opinion among roadbuilders 
as to the advisability of using stone of uniform size. 
Some insist quite strenuously upon this point and care- 



BROKEN-STONE ROADS. I47 

fully screen their stone with the object of getting it as 
uniform as possible; while others declare that the varia- 
tion of size is an advantage, and even that the stone 
should not be screened after coming from the crusher, 
except to remove the stone above the limiting size and 
to get rid of dust and foreign matter. 

Uniformity of size probably makes the wear more 
even, but the presence of smjaller fragments facilitates 
the binding together of the material. If the varying 
sizes bs well distributed through the mass of stone, the 
variation of size has the advantage of lessening the amount 
of voids, and makes it possible to compact the stone 
in the road with a less quantity of binder. Screening 
out the fine parts and dust eliminates the danger of having 
portions of the road made up entirely of fine material, 
and secures a proper distribution of the binder through 
the mass of stone. 

The lower courses of stone in the road may be of any 
sizes which are most convenient, provided the stones 
are not too large to become firmly compacted under the 
roller. WTien the stones of the surface layer are small 
in size, it is common to use the larger sizes in the bottom 
course, thus making it unnecessary to break all to the 
small dimension. 

If the surface of a road is to be constructed of very 
hard rock the stones for the surface layer may include 
those from about h inch to ij inches (or at most i^ inches) 
in diameter with good results. In the work of the 
Massachusetts Highway Commission: " All broken stone 
used is separated into three sizes by passing it through a 
screen with meshes | inch, i§ inches, and 2 J inches in 
diameter. The largest size is placed at the bottom and is 
covered with the successive smaller sizes. The different 
sizes of stone are spread in courses. The sub-grade and 



148 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

each course of stone are rolled thoroughly, and the top 
course is watered before rolling." 

In constructing roads with limestones, it is often desir- 
able to use larger materials for the surface layer, as these 
offer better resistance to the wear of trafhc. For such 
roads the surface may be composed of stones from about 
I J, or if inches to 3, or even 3 J inches in diameter; the 
smaller sizes being used in the bottom course of the road, 
and th9 screenings for binder. Stone of this character 
may also be advantageously used by making both courses 
of material containing a greater range of sizes, as in the 
specifications of the Illinois Highway Commission for 
19 II, which require that: 

" Sizes: Two sizes of crushed stone shall be used as 
follows: (a) Broken to a size that will pass over a i-inch 
ring, and through a 2j-inch ring, which size will herein- 
after be referred to as 2j-inch stone, (b) Broken to a size 
that will pass through a i-inch ring and graded to a dust, 
which size will hereinafter be referred to as screenings. 

" The first course of stone shall be 2j-inch broken 
stone spread to compact under rolling to the thickness 
shown on the plans. 

•' After the first course of stone has been spread, it 
shall be harrowed with a stiff-tooth harrow (having a 
weight of 10 to 12 pounds per tooth) until a uniform size 
of stone is brought to the surface and all fine material 
which may have been mixed with the 2j-inch stone has 
been shaken to the bottom of the layer of stone. 

" After the broken stone for the first course has been 
spread to a uniform thickness and harrowed, and has a 
proper cross-section, it is to be rolled with a steam roller 
weighing not less than 10 tons, until it is compacted 
to form a firm, smooth surface. The rolhng must 
begin at the sides and work towards the center and 



BROKEN-STONE ROADS. 149 

the rear wheels of the roller must cover this space 
thoroughly." 

The second course of stone is spread in the same manner 
as the first, after which the filler is applied, as follows: 

" After the second course of stone has been rolled and 
completed as specified, the screenings are to be spread, 
but in no case are screenings to be used until the second 
course has been thoroughly rolled and compacted. The 
screenings are to be spread dry with shovels from piles 
along the road, or from dumping boards, but in no case 
are the screenings to be dumped directly on the second 
course. The quantity of screenings used is to be such 
as will just cover the second course of stone. 

"After the screenings are spread they are to be sprinkled 
with water from a properly constructed sprinkling cart 
and then rolled with a steam roller weighing not less 
than. 10 tons. The amount of water to be used to be 
determined by the engineer. The rolling is to begin 
at the sides and to continue until the surface is hard and 
smooth and shows no perceptible tracks. The rolling 
and watering shall continue until the water flushes to 
the surface. 

'^ If after rolhng the screenings, the stone appears 
at the surface, additional screenings shall be used in such 
places. The rolling is to extend over the whole width 
of the macadam." 

Gravel is frequently used for roads constructed in 
the same manner as with broken stone, both with and 
without the telford foundation. The requirements of 
a good gravel for this purpose are the same as for a 
good stone. The stones of the gravel should be sharp 
and angular, and must possess the qualities of hardness 
and toughness. Water-worn material is therefore ob- 
jectionable, as it will not compact without the use of 



150 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

large amounts of soft binding material. In many 
places a hard flint gravel occurs which is not inferior to 
the best broken stone. This frequently occurs when 
the available rock is soft limestone and may be used 
to advantage as a surface upon a base of the soft rock. 

Gravel should be screened to remove the larger stones 
and the fine material, and then treated in the same 
manner as broken stone. 

Gravel not fit for surface material may often be used 
to advantage as a base under a surface of hard rock; 
in many instances, economy would result from the 
substitution of gravel for broken stone in such work. 
Slag and cinders may also sometimes be used in the 
same manner. 

In cases where local stone is being used for the lower 
courses of a road to be surfaced with trap or other 
more durable rock, or where a flint gravel surface is 
used upon a soft limestone base, the screenings from 
the stone used in the lower layer may often advan- 
tageously be used in binding the surface, the whole 
run of the crusher except the screenings through about 
a one-half inch screen being used in the lower course of 
the road. 

In some instances the binding material is mixed with 
the surface stone before placing upon the road. The 
following extract from the specifications of Mr. James 
Owen for roads in Essex County, New Jersey, shows 
this practice: ''When the two courses are rolled to 
the satisfaction of the Engineer and Supervisor, a coat 
of fifty (50) per cent of three-quarters (|) inch stone 
and fifty (50) per cent of screenings property mixed 
is to be spread of sufficient thickness to make a smooth 
and uniform surface to the road; then again rolled 
until the road becomes thoroughly consolidated, hard 



BROKEN-STONE ROADS. 151 

and smooth. " This specification is remarkable for 
the large quantit}^ of screenings used, and needs great 
care in securing a proper mixture of the two materials. 

Art. 39, Compacting the Road. 

The materials may be compacted in a road either by 
placing them in position and allowing the traffic to 
pass over them or by rolling with a steam or horse 
roller. 

The first method by itself is seldom practiced when 
it is possible to avoid it. It is hard upon the traffic, 
takes a long time to reduce the road to compact con- 
dition, and a smooth surface is with difficulty pro- 
duced. Where heavy horse rollers are employed they 
are clums3^ and inconvenient to handle, and the work 
of rolling is slow^ as compared with the steam roller. 
In many instances, however, good results are obtained 
with them. They are not so expensive in first cost as 
steam rollers, and have not the disadvantage of fright- 
ening horses. 

Horse rollers are usually arranged so that the direc- 
tion of motion may be reversed without turning the 
roller itself around, and also so that the weight ma}^ be 
changed bj^ placing additional weight inside. the roller 
or removing it. Horse rollers for this purpose usually 
bring a pressure of from 125 to 250 pounds per linear 
inch upon the road and weigh from 3 to 6 tons. 

Steam rollers w^eighing from 8 to 15 tons are most 
commonly employed for compacting the road mate- 
rials. They have the advantage of forcing the materials 
at once into a firm and compact mass and producing a 
smooth surface for the immediate use of travel. They 



152 A TEXT-BOOK ON ROADS AND PAVEMENTS 

admit also of the use of hard materials for binding. 
These rollers give a pressure under the drivers of from 
400 to 650 pounds per linear inch. 

The stone forming the body of the road should be 
placed and partially compacted before the addition of 
the small material, which may then be worked into the 
spaces between them. 

The office of the binding material is to hold the 
stones in place and form a bearing for them, as well as 
to prevent the passage of water between them. It has 
no duty to perform in sustaining the loads. This is 
the objection to having the binding material mixed 
with the stones in advance, as would be the case when 
unscreened stone is used. A portion of the road stones 
would be replaced by small material instead of having 
this material only in such voids as necessarily exist be- 
tween the stones. 

The quantity of binding to be used is that which 
will be barely sufficient to fill all the voids in the larger 
material. It has been contended that the lower por- 
tion of the road should be porous in order to facilitate 
the escape of any water that may find its way through 
the surface, but the tendency of the best modern prac- 
tice is in the direction of filling all the voids as nearly 
as possible, thus making the entire road practically one 
solid body, and it is now commonly agreed that the sur- 
face of a properly constructed broken-stone road is very 
nearly impervious to water. 

The voids in loose broken stone comprise about 40 
to 50 per cent of the volume. In the stone when 
compacted in the road the voids are somewhat reduced, 
probably ranging from 30 to 40 per cent of the volume. 
The voids may be approximately determined in any 



BROKEN-STONE ROADS. 1 53 

case by filling a measure with the stone, shaken down 
as closelj^ as possible, and then measuring the quantity 
of sand that can be added in the same manner. 

In constructing a road with the use of a steam-roller, 
the road-stone is first put on to the required thickness 
and the roller passed over it to settle the stones into 
place and reduce the voids as much as possible. The 
binding material, representing a volume about equal to 
the voids in the stone, is then added, sprinkled, and 
rolled until the small material is washed and forced into 
the interstices, giving a smooth, hard surface. This is 
repeated for each layer of stone, or in some cases the 
small material is applied only to the top layer. 

A thin coating of the binding material is then spread 
upon the surface and the road thrown open for travel. 

Art. 40. Thickness of Road-covering. 

The thickness necessary for a road-covering depends 
upon the amount of the traffic it is to bear and upon the 
nature of the foundation afforded by the road-bed. 
Under a heavy traffic it is advisable to make the road- 
covering heavier than might be allowable for lighter 
traffic, in order to provide for wear and lessen cost of 
renewals. 

When the road-bed is firm, well drained, and not 
likely to soften at a wet season, it will always afford a 
firm bearing, upon which the covering may rest. The 
loads coming upon the road are then simply transmitted 
through the covering to the road-bed beneath, and 
there is no tendency on the part of the loads to break 
through the covering other than by direct crushing of 
its material. If, however, the road-bed may become 
soft in wet weather, it will then lose its power to firmly 



154 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

sustain the covering at all points, and the covering 
must possess sufficient strength to bridge over places 
where it is not supported from beneath, or a load com- 
ing upon it may break through by bending it down- 
ward at such point. The thickness of road-covering, 
therefore, must be greater where the road-bed is less 
perfect. 

The intensity of freezing that may be expected also 
has an influence upon the necessary thickness of the 
road-covering. The effect of frost upon the road will 
depend in large measure upon the condition of the 
road-bed, and thus make the thickness depend in still 
greater measure upon its nature. Freezing will not 
injure a dry road-bed, but if it be damp and have but 
a thin covering the road is likely to blow or be thrown 
up by the action of frost. 

For roads on considerable grades the thickness of the 
road-covering is often reduced below what is used on 
flat ones, because of the better drainage afforded by 
the slopes. It is to be remarked, however, that if the 
slopes are very steep the wear of the surface becomes 
so great, due to the horses' efforts to obtain foothold 
and to the washing of surface-waters during rains, 
that the thickness of the coating should be increased. 

Macadam roads are commonly made from 4 to 12 
inches thick, and telford roads from 8 to 12 inches, of 
which 5 to 8 inches may be foundation pavement. 

A covering 6 to 8 inches thick is usually sufficient for 
nearly any case of a country road, unless laid upon bad 
foundation, or to carry exceptionally heavy traffic. 
When the road-bed is formed of firm material and w^ell 
drained, a covering of 4 or 5 inches of broken stone or 
gravel may give good service under considerable trafiic. 

A thin road to be effective must have its interstices 



BROKEN-STONE ROADS 155 

well filled with binding material and be thoroughly 
compacted by rolling. It will then present no voids to 
be filled by the soil pressing upward from below, and 
at the same time it will be practically impervious and 
prevent surface-water from reaching the road-bed, thus 
keeping the material in good con^lition to sustain the 
loads. The 4-inch roads of Bridgeport, Conn., which 
are often cited as examples of successful work, are con- 
structed in this manner of exceptionally good mate- 
riaL In other cases where thin roads have proved fail- 
ures the trouble may often be traced to dampness in 
the subsoil or to lack of thorough construction. 

Instances will frequently be met in practice where a 
road must be constructed over material which is likely 
to be unstable and cannot be made firm by drainage. 
In such cases, thick roads must be built. Where the 
conditions are unfavorable, a road 12 to 16 inches thick 
ma}^ be necessary. 

In many cases the problem to decide, in determining 
the thickness of a covering, is whether to use heavy 
construction or thorough drainage. It is easier to get 
good results with thick road-coverings, and they are 
in general safer to use; but skillful adaptation of less 
material may often save expense in construction with 
good results. The peculiar conditions of each case must 
decide what is best for that case. 

On country roads the macadam surface should be 
given a crown of from one-thirtieth to one-twenty- 
fourth of the width in order to provide good drainage. 
In manyinstances a considerable saving in road material 
iJiSLY be effected b^^ making the road thinner at the 
edges than in the middle. The Massachusetts Highway 
Commission in some instances reduce the thickness of 
their 6-inch roads to 2 7 or 3^ inches at the edges. 



156 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Some engineers grade the road-bed without leaving a 
bench at the side, and reduce the stone to thin edges. 
It is doubtful if there is any economy in this practice, 
as it is wasteful in the use of stone, although it effects 
a small saving in the cost of grading the road-bed. 

Art. 41. Maintenance of Broken-stone Roads. 

To maintain a broken-stone road in good condition 
it is necessary first of all that it be frequently cleaned 
of mud and dust, and that the gutters and surface 
drains be kept open to insure the prompt discharge 
of all water that may come upon the surface of the road. 

The best method of making repairs that may become 
necessary to the road-surface depends upon the char- 
acter of the material composing the surface and the 
weight of the traffic passing over it. 

If the road metal be of soft material which wears 
easily, it will require constant supervision and small 
repairs whenever a rut or depression may appear. 
Material of this kind binds readily with new material 
that may be added, and may in this manner frequently 
be kept in good condition without great difficulty, while 
if'not attended to at once when wear begins to show^ it 
will very rapidly increase, to the great detriment of the 
road. In making repairs by this method, the material 
is commonly placed a little at a time and compacted by 
the traffic. The material used for this purpose should 
be the same as that of the road-surface, and not fine 
material which would soon reduce to powder under the 
loads which come upon it. By careful attention to 
minute repairs in this manner a surface may be kept 
in good condition until it wears so thin as to require 
renewal. 



BROKEN-STONE ROADS. 157 

In case the road be of harder material that will not 
so readih' combine when a thin coating is added, the 
repairs may not be so frequent, as the surface will not 
wear so rapidly and immediate attention is not so 
important. It is usually more satisfactory in this case 
to make more extensive repairs at one time, as a larger 
quantity of material added at once may be more readily 
compacted to a uniform surface, the repairs taking the 
form of an additional layer upon the road. 

AA^here the material of the road-surface is very hard 
and durable, a well-constructed road may wear quite 
evenly and require very little, if anything, in the way 
of ordinary?- small repairs until worn out. It is now 
usually considered the best practice to leave such a 
road to itself until it wears very thin, and then renew it 
by an entirely new layer of broken stone placed in the 
same manner as in original construction, on top of the 
worn surface, and without in any wa^^ disturbing that 
surface. If a thin layer only of material is to be 
added at one time^ in order that it may unite firmly 
with the upper layer of the road it is usually necessary 
to break the bond of the surface material before plac- 
ing the new layer, either by picking it up bj^ hand or, 
if a steam roller is in use, by means of short spikes in 
its surface. Care should be taken in doing this, how- 
ever, that only the surface layer be loosened, and that 
the solidity of the body of the road be not disturbed, 
as might be the case if the spikes are too long. 

Much difficulty has also been experienced in some 
localities, where the macadam roads connect with earth 
roads which in wet weather are composed of heavy, 
sticky mud, on account of the '' picking up " of the 
macadam surface in muddy weather by the wheels 
of vehicles which are covered with mud. The stones 



15S A TEXT-BOOK ON ROADS AND PAVEMENTS. 

in the surface are loosened and carried off until the road 
is destroyed. This has given much difficulty in some of 
the states of the middle west, where limestone macadam 
is used. This trouble is reduced by forming the surface 
of large materials but considerable strength is essential 
in the binding material to successfully resist destruction 
from this cause. 

The maintenance of macadam roads under trying condi- 
tions or under severe traffic has in many instances proven 
a matter of considerable difficulty and of large expense. 
Under ordinary circumstances the destruction of a broken 
stone road is greatest in dry and dusty weather. If the 
road is subject to considerable travel, wear becomes 
rapid and a certain amount of the road metal is blown 
away by the wind, washed away in case of rain, or cleaned 
from the surface as mud. The binding material wear- 
ing into dust and being removed from the road loosens 
the stones of the road-surface, causing the road to " ravel." 

To protect a broken-stone road against excessive 
wear and prevent raveling in dry weather, some means 
of laying the dust must be used. Sprinkling the road- 
surface with water is often used for this purpose 
and has an important effect in reducing the wear and 
prolonging the life of the road. If the road be sys- 
tematically sprinkled, the material ground off by the 
traffic will pack upon the surface, forming a cushion 
which serves to protect it from further attrition. In 
sprinkling, the object should be to keep the surface damp, 
and not to flood it by applying too large a quantity of 
water at once. 

The recent great increase in the extent of automobile 
travel upon country roads has introduced a new element 
into the problem of road construction and maintenance. 
The destructive effect of these rapidly moving vehicles 



BROKEN-STONE ROADS. 1 59 

IS such that, under any considerable traffic of this char- 
acter, an ordinary macadam road is quickly destroyed, 
and special methods of construction, or of maintenance, 
must be employed if satisfactory results are to be obtained. 
The driving ^Yheels of the automobile produce a back- 
ward thrust upon the surface of the road which tends to 
remove the binder and loosen the stones composing 
the surface, while the rapid motion of the body of the 
car causes air currents which draw the dust from the 
road surface and throw it upward behind the automobile. 
The maintenance of country roads against the destruc- 
tive effect of automobile travel is largely a question of 
preventing the formation of dust upon the surface of 
the road. Sprinkling with water may serve in towns, 
but is expensive and not apphcable on country roads. 
The use of oil in earth-road construction has been dis- 
cussed in Art. 29, and a number of methods have been 
proposed for eliminating dust on broken-stone roads; 
these will be discussed in Art. 42. 



Art. 42. Dust Prevention. 

The excessive production of dust upon country high- 
ways of large traffic presents, to the road engineer, one 
of the most important problems with which he has to 
deal. This dust is a source of discomfort to people 
using the road, or living in its vicinity, a menace to the 
health of those breathing the dust-laden air, and causes 
the quick destruction of the surface of the road itself. 

The discomforts caused by road dust are evident to 
all observers. It penetrates into the houses, damaging 
furnishings and causing increased labor to the house- 
keeper, injures clothing, and prevents the enjoyment 



l6o A TEXT-BOOK ON ROADS AND PAVEMENTS, 

of lawns and porches. Vegetation in the vicinity of the 
road is often injured and fruit destroyed. 

The effect of dust upon the heakh of men and animals 
is not yet fully understood, but enough is known to indicate 
that dust is an efficient agent for the dissemination of 
diseases which are communicated by germs. The 
ehmination of dust is considered of special importance 
in the effort to eradicate tuberculosis. 

Road dust consists of very finely divided particles of 
material abraded from the surface of the road by the 
traffic, which, when in a dry condition, are easily carried 
away by the winds or distributed through the air by 
passing vehicles. The extent to which dust is formed 
depends therefore upon the resistance of the road metal 
to abrasion and the tenacity of the binder used to hold 
the stones together, as well as upon the extent to which 
the winds and traffic remove the dust from the road 
surface. If the metal abraded by the traffic would 
remain upon the surface of the road it would serve to 
protect the surface from further abrasion, and prolong 
the life of the road This material, however, is rapidly 
removed as dust in dry weather, or washed away by 
rain, leaving the surface exposed to further abrasion. 

METHODS OF DUST PREVENTION. 

The formation of dust may be lessened by so constructing 
the road as to cause the surface to offer greater resistance 
to abrasion and shear of the traffic, or by moistening the 
surface so as to prevent the particles abraded from 
flying into the air, and cause them to adhere to each other 
and to readily pack upon the surface. We therefore 
distinguish between the treatment applied to ordinary 
macadam surfaces for the purpose of laying dust, and the 



BROKEN-STONE ROADS. l6l 

construction of macadam roads with special binders, 
although the two methods have much the same objects 
in view. 

The construction of broken-stone roads with special 
binders will be discussed in a separate chapter (see 
Chap. VI) under the head of Bituminous Macadam. 
The use of oil upon earth roads for the same purpose 
has been discussed in Art. 29; here we will consider 
only the methods used for laying dust upon ordinary 
macadam surfaces. 

Numerous materials have been proposed for use as 
dust layers and many experiments have been made for 
the purpose of determining their efficiency in use. Com- 
paratively few of these are of much importance and in 
consequence of the varying conditions under which the 
experiments have been made and the different methods 
of application, it is somewhat difficult to arrive at a 
definite conclusion as to the relative values of the dif- 
ferent materials or the best way of applying them. 

Water. As mentioned in Art. 41, the use of water for 
sprinkling the surfaces of broken-stone roads not only 
lays the dust, but has a marked effect upon the Hfe of 
the road by protecting the surface from further abrasion. 
The use of water for this purpose has, however, the dis- 
advantage of requiring frequent sprinkling on account 
of the rapid evaporation of the water in dry w^eather, 
and is on this account not readily applicable on country 
roads, where water is not easily available. For the streets 
of towns, sprinkling is still very commonly employed, 
but it seems probable that in most instances, upon macadam 
streets, the amount of labor required is such that a treat- 
ment of more permanent character would be cheaper. 

Calcium Chloride. The use of calcium chloride 
(CaCla) is based upon its hygroscopic and deliquescent 



l62 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

properties. When sprinkled upon the road surface it 
retains water, or absorbs water from the air, and hquefies, 
thus keeping the surface moist and preventing the abraded 
material from drying into dust. 

Calcium chloride may be obtained either in a solid 
granular condition or in a concentrated solution. It 
is produced as a by-product in the preparation of bicar- 
bonate of soda from common salt by the use of ammonia. 
When the material must be transported to considerable 
distances, it is desirable to obtain it in solid form on 
account of the cost of transportation, but otherwise the 
solution is convenient as saving the trouble of dissolving 
the salt. 

In applying calcium chloride to a road surface, it is 
customary to use a solution containing from about 8 to 
20 per cent of the salt, and distribute it from an ordinary 
sprinkling cart. In the first application, the more con- 
centrated solution (perhaps 15 per cent) may be used, and 
when other applications become necessary a more dilute 
solution (8 per cent to 10 per cent) is employed. If about 
600 gallons of the solution be sprinkled on 1000 feet in 
length of an 18 feet width of road, from \ to tJ pound of 
the calcium chloride is required per square yard of road 
surface In England, somewhat larger quantities than this 
have commonly been employed, about f pound being used 
for the first application, with subsequent applications 
of i to J pound. On some American roads, appKcations 
of less quantities have been found fairly satisfactory, 
about I pound being used for the first, and to or | pound 
for subsequent applications. 

When the calcium chloride is obtained in solid form it 
should be dissolved, forming a concentrated solution (40 
per cent) in advance of the time when it is to be used, and 
placed in tanks at points where water may most conven- 



BROKEN-STONE ROADS. 163 

iently be obtained, for diluting it, near the road to be 
treated. The salt is readily soluble in water. If several 
hours may be allowed for the dissolving to take place, 
it may be suspended in a wire basket under the surface 
of the water in the tank until solution is complete, but, 
if it must be dissolved for immediate use, mechanical 
agitation must be employed to hasten the process. 

In the humid climate of England this method has been 
employed for laying dust with good results, applications 
being made at intervals of six weeks or two months dur- 
ing the summer and fall. In the drier climate of the 
United States, the use of calcium chloride has, in some 
instances, been found economical as a substitute for 
sprinkhng with water, although it has been found neces- 
sary when the air is dry to supply water to the salt by 
occasional sprinkling, sufficient water not being absorbed 
from the air during the hot part of the day. This method 
is not applicable to ordinary country roads in the United 
States, and can only be used at reasonable cost where 
water is available along the line of the road for convenient 
use in sprinkling. 

The application of calcium chloride has no permanent 
effect on the road, and is gradually washed away by the 
rain and must be renewed several times during the 
season. It is odorless and clean and, when the atmosphere 
is sufficiently humid to supply the necessary moisture, 
is a good dust layer. On residence streets or suburban 
roads, where water is available, it may be found a satis- 
factory means of preventing dust, although the cost 
may be somewhat greater than some of the other methods 
used for that purpose. In several instances it has been 
used at a cost of about 2 cents to 4 cents per square yard 
of road surface per annum, with the calcium chloride 
about Si 5 per ton. 



1 64 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

* In regard to ascertaining and regulating the strength 
of the solution, the most convenient method is to deter- 
mine its specific gravity by means of a hydrometer. 
Accurate determination have been made of the specific 
gravity of solutions of known percentage compositions, 
and, as hydrometers graduated to direct specific gravity 
readings can be obtained, the method is a very simple 
one. A hydrometer graduated from i to 1.4 is most 
suitable for ordinary work, and by comparing the readings 
with the following table, the strength of solution at 15 
C. can be immediately ascertained. Also by diluting 
the salt or concentrated solution with water, any desired 
strength may be obtained if the dilution is stopped at 
the specific gravity indicated for that particular strength. 

Per cent calcium chloride 5 8 10 15 20 30 40 

Specific gravity 1.041 1.068 1.086 1.132 1.182 1.286 1.402 

Sea-water. Attempts have frequently been made 
to use sea- water for dust prevention, with the object of 
reducing the number of sprinklings necessary, and the 
cost of laying the dust, through taking advantage of the 
presence of certain hygroscopic and deliquescent salts 
in the sea-water. The reliance is mainly upon magnesium 
chloride (MgClg) which is always found in the sea- water, 
and which possesses the desired properties to a somewhat 
less degree than calcium chloride. It has been found 
that the effect of sprinkling is more lasting than when 
fresh water is used and the number of sprinklings may 
be lessened, but the presence in the water of other salts 
not possessing hygroscopic properties, and which cause 
disagreeable mud in wet weather, have rendered this 
treatment, in some instances, rather unsatisfactory. 
It has been claimed that this salt mud is injurious to 

* Hubbard, Dust Preventives and Road Binders, New York, 19 10. 



BROKEN-STONE ROADS. 1 65 

horses' feet, and destructive to the iron work of 
vehicles. 

Oil. Bituminous materials have recently come into 
extensive use as substitutes for water in laying the dust 
upon macadam roads. These may be used as tem- 
porary dust preventives or as permanent road binders. 
These materials will be discussed in detail in Chapter 
VI. The asphalts and tar products are not usually 
employed as temporary dust preventives, but the petro- 
leums are frequently used for the purpose. Many dif- 
ferent grades of oil have been tried with varying degrees 
of success. 

Crude Petroleums are very commonly employed. Those 
having an asphaltic base, like the California oils, give the 
best results. The heavier parts of these oils have bind- 
ing properties which exert a lasting effect upon the road, 
when the more volatile portions have disappeared. The 
petroleums with paraffine bases, however, like the oils 
from the Pennsylvania district,possess no binding properties 
and sometimes produce an objectionable slime which 
makes them worse than useless. The semi-asphaltic 
oils from the Texas and Kansas fields have also been 
fairly successful, but residual oils obtained from these 
mid-continent petroleums, after the removal of the 
lighter parts, have been found much more satisfactory, 
and seem to be the best material available for ordinary 
sprinkling in the Eastern States, where the cost of trans- 
portation would prevent the use of California petroleum. 

The oil is easily distributed upon the surface of the road 
by means of the ordinary sprinkling wagon, i to ^ gallon 
being required per square yard of surface. One or two 
applications may be needed each season, depending 
upon the character of traffic upon the road, and the 
amount and character of oil applied. With oil costing 



l66 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

4 to 5 cents per gallon, the cost of treatment may vary 
from about f to ij cents per square yard of road surface. 
This will usually be considerably cheaper than sprinkling 
with water or the use of calcium chloride. Oil should 
not be applied to a surface covered with dust, but the 
road should be clean and dry when the oil is applied, 
and dry weather immediately following the application 
is desirable also, in order to lessen the somewhat objec- 
tionable period during which it is being absorbed into 
the surface. 

Oil Emulsions. Various methods have been devised for 
using oils in the shape of emulsions, with a view to reduc- 
ing the difficulties and costs of applying the oil to the 
surface of the road. There are quite a number of processes 
under various names, some of them patented, and most 
of them depending upon the use of ammonia or soap 
solutions as emulsifiers. A machine has also been 
invented for the purpose of forming emulsions mechan- 
ically, without the use of saponifying materials. This 
is known as the " Emulsifix," and consists of a wagon 
carrying two tanks, one for the oil and the other for 
water. These are connected by pipes with a third 
tank, in which the mixture is formed by rapidly revolving 
blades, which also force the mixture upon the road sur- 
face in a line spray. The water soon disappears leaving 
the oil in a finely divided state over the surface to act as 
a dust layer. 

The effect of oil emulsion is temporary and they need 
to be applied several times during the season. The 
expense may be somewhat greater than where a single 
application of oil is made, but the road may be used 
immediately following the application, without the 
objectionable conditions which follow the use of a larger 
quantity of oil. Heavier oils may be used as emulsions 



BROKEN-STONE ROADS. 167 

than could be sprinkled cold without the water, and these 
may act as binders in the road surface after a number 
of applications, with more lasting effect than the lighter 
oils. 

Chemical emulsions are on the market in concentrated 
form. These are prepared so as to be readily miscible 
with water at the site of the work before using. When 
these contain a good oil of asphaltic base they frequently 
give good service. 



CHAPTER VI. 
BITUMINOUS MACADAM ROADS. 

Art. 43. Types of Bituminous Macadam. 

The use of oil upon macadam roads as a means of 
dust laying has already been considered in Art. 42. 
Such use has for its object the preservation of the road 
through preventing the formation of dust. This effect 
is temporary and needs to be frequently repeated, leaving 
no permanent binder in the road surface. In the con- 
struction of bituminous macadam, however, the purpose 
is to introduce bitumen into the road surface as a binder, 
in order to increase resistance to the wear of traffic, by 
cementing the surface metal firmly together, and im- 
pregnating it with bitumen. 

The methods employed in constructing bituminous 
macadam roads are of several different types: 

(a) Surface Treatment, which consists in applying 
a coating of bitumen to a finished surface of water-bound 
macadam. This coating is applied either cold or heated, 
and is usually covered with a layer of sand or other fine 
material, and rolled. Two applications are commonly 
made, the object being to secure an even coating over 
the road, and to permit the bitumen to be absorbed into 
the road material to as great an extent as possible. 

This treatment is for the most part limited to the treat- 
ment of roads already built, although sometimes employed 
in new construction. For old roads which are in good 
surface, it forms a convenient method of treatment, 

168 



BITUMINOUS MACADAM ROADS. 1 69 

without renewing the surface. The details of the method 
are discussed in Art. 49. 

(b) Penetration Method. In constructing a bituminous 
macadam surface by this method, the macadam is first 
placed in the ordinary manner, but without applying the 
binder to the surface layer of stone. A coating of bitumen 
is then given to the surface, and allowed to flow into 
the voids in the stone, after which stone chips are applied 
and rolled into the surface. A second coating of bitumen 
is usually applied and covered with additional chips, or 
screenings, and the whole rolled to a smooth surface. 

This method is extensively employed in the United 
States, the materials used and the details of construction 
varying widely in different parts of the country. These 
are discussed in Art. 50. 

(c) Mixing Method. This consists of using for the 
surface layer, bituminous concrete, obtained by mixing 
hot bituminous binder with the macadam stone. The 
lower course of the road is usually formed of water-bound 
macadam, the surface material, which has been previously 
mixed, is then placed to the proper thickness and rolled 
to a smooth surface. Sometimes the stone is heated 
before mixing with the bitumen, and sometimes used 
cold. Commonly a layer of stone chips is rolled into 
the surface of the concrete, and frequently a paint coat 
of bitumen is also applied, covered with sand or stone 
chips, and rolled to a finished surface. 

This method is largely used in England and to a less 
extent in the United States. It is applied mainly to 
new construction, although sometimes used in resurfac- 
ing old roads. The methods used in construction are 
discussed in Art. 51. 

In the construction of city streets, bituminous con- 
crete is frequently used, which is obtained by more care- 



lyo A TEXT-BOOK ON ROADS AND PAVEMENTS. 

ful and elaborate methods, closely graded aggregates 
being employed, sometimes with cement concrete founda- 
tion. This more expensive construction is not classed 
under the head of bituminous macadam, but is commonly 
known as bitulithic pavement and is discussed in Chap- 
ter IX. 

(d) Gladwell Method. This method was developed 
in England as a means of resurfacing old roads, but is 
also used in new construction. It has not been used 
to any extent in the United States. 

A foundation coarse of water-bound macadam is 
first formed, or the surface of the existing macadam is 
smoothed and cleaned. A light layer (about f inch) 
of tarred chippings is then spread, and upon this the 
road metal is placed and rolled into the chippings. On 
top of the macadam surface so formed, a second layer 
of the tarred chippings is spread and rolled into the voids 
in the surface stone, with the object of thoroughly filling 
the macadam stone with the matrix. The surface is 
then sealed by a light coating of hot tar preparation, 
which is covered with screenings and rolled. 

According to the specifications of the inventors of the 
method, the matrix is composed of clean dry granite 
screenings mixed warm with a special tar preparation, 
while the aggregate is broken to nearly uniform size 
(2 ins. to 2 J ins.). The good results which have been ob- 
tained with these roads in England, have been attributed* 
to the excellence of the materials used in forming the 
matrix rather than to the method of construction. 

(e) Rock Asphalt Macadam. The method of con- 
structing these roads is to form the macadam surface 
in the same manner as for water-bound macadam, and 
then, in place of the usual binding material, to apply 

* Smith, Dustless Roads, Tar Macadam, London, igog. 



BITUMINOUS MACAD.\aM ROADS. 171 

a top dressing of ground rock asphalt and roll to a smooth 
surface. The surface layer of stone is thoroughly rolled, 
without binder, to a thickness of about 2 J or 3 inches, 
this is then covered with a thin layer, about h inch, of 
the rock asphalt, which is rolled thoroughly into the 
voids in the stone. A thicker layer, about i inch, of the 
rock asphalt is then placed and rolled to a smooth sur- 
face. The stone for the surface layer is usually of rather 
uniform size, i inch to 2 inches, and must be quite dry 
when used. A thin cushion of the rock asphalt should 
be left over the surface upon completion, to be forced 
into the voids in the surface metal under the action of 
traffic, although it is desirable that the wear of traffic 
come upon the stone of the surface course and the asphalt 
serve purely as binder. 

When the asphalt is convenient to the work and the 
cost of transportation is not too great, these roads may 
often be economically constructed and give good ser\dce. 
Rock asphalt for this purpose should contain about 
7 to 10 per cent of bitumen. Kentucky asphalt has 
been used to considerable extent for this purpose very 
successfully. 

Art. 44. BiTUi^nNOUs Materials. 

The term bitumen is used to designate a class of sub- 
stances, consisting of a mixture of various series of hydro- 
carbons and possessing certain physical and chemical 
properties by which they are defined. Several groups 
of hydrocarbons are commonly present in each sample 
of bitumen, and the range of possible mixtures is very 
great, so that considerable confusion exists concerning the 

assification of the various substances included under 
this designation, as well as in defining the limits within 
which substances may be considered to be bitumens. 



172 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Mr. Prevost Hubbard has made a very careful study of 
these materials, and gives the following definitions : * 
" Bitumens may be described as consisting of a mixture 
of native or pyrogenetic hydrocarbons and their derivatives, 
which may be gaseous, liquid or solid, but if solid, melt- 
ing more or less readily upon the application of heat, 
and soluble in chloroform, carbon bisulphide and similar 
solvents." 

Bitumens being thus a mixture of various hydrocarbons 
in differing proportions, have no fixed compositions, 
and vary widely in properties according to the characters 
and amounts of the hydrocarbons of which they are 
composed. They are divided into natural bitumens 
and artificial bitumens. Natural bitumens are those 
which occur in nature as mineral bitumeas. Artificial 
bitumens are those which are formed by the distillation of 
certain other mineral substances known as pyrobitumens. 
The distillation of natural bitumens also gives rise to 
other bitumens of differing characteristics, and these, 
particularly those left as residues after distillation, are also 
designated artificial bitumens. 

NATIVE BITUMENS. 

Bitumens occur in nature as natural gases, petroleums, 
malthas and asphalts, beside a number of other materials 
which gradually merge into the pyrobitumens. 

Petroleums are liquid bitumens and are divided into 
paraffine oils, cylic or asphaltic oils and semi-asphaltic 
oils. The paraffine petroleums consist mainly of the 
paraffine hydrocarbons (CnH2n + 2) and are of Httle 
importance as road materials, showing no permanence 
as dust preventives and no binding properties in the road. 
The asphaltic petroleums are characterized by the poly- 

* Dust Preventives and Road Binders, New York, 1910. 



BITUMINOUS MACADAjM ROADS. 1 73 

methylene hydrocarbons and when distilled yield a residue 
similar in character to asphalt. The semi-asphatic oils 
consist of a mixture of the paraffine hydrocarbons with 
those of the asphaltic oils. 

Asphalts are solid bitumens composed of hydrocarbons 
of the same characters as those of the asphaltic oils, but 
with the lighter and more volatile parts remoA^d. This 
class includes the true asphalts and a number of similar 
materials, such as glance pitch, manjak and gilsonite. 

Malthas are heavy oils intermediate between the asphal- 
tic petroleums and the asphalts. They are similar in 
character to the fluid residuums derived from asphaltic 
petroleums, but, as they contain more of the volatile 
hydrocarbons, may be hardened by exposure to the air, 
or by heat, becoming an artificial asphalt. These mate- 
rials have been produced to some extent in California and 
seem to form desirable road materials. 

ARTIFICIAL BITUMENS. 

The artificial bitumens which are of Importance as 
road materials are crude tars, residues from die dis- 
tillation of petroleums and residues from the distillation 
of tars. The crude tars include water-gas tars, and coal 
tars. Residues from the distillation of petroleum are 
either residual oils or residual pitches. The residual 
oils are obtained by removing all of the more volatile 
oils, including the lubricating oih. These may be used 
as road oils, when derived from asphaltic petroleum, 
and are also employed as fluxing agents ia preparing 
asphalts for use in pavements. Residual pitches are 
obtained when distillation is carried far enough to leave 
a solid residue. If derived from asphaltic oil these may 
give good road materials. 



174 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

The residues derived from the distillation of tars are 
known as dehydrated tars, refined tars and tar pitches. 
Dehydrated tar is that from which the water has been 
removed. Tar pitches are formed when distillation 
leaves solid or semi-solid residues. Refined tars are 
products intermediate between dehydrated tars and 
pitches. All of these materials may be of use as road 
binders when carefully prepared from coal-tar. 

Art. 45. Petroleums. 

Petroleums, or mineral oils, are widely distributed 
throughout the earth. They occur in generous quantities 
in many parts of the United States, the oils of different 
localities varying widely in character. 

The Applachian oil field lies on the v/est side of the 
Alleghany Mountains, through Western Pennsylvania, 
West Virginia, Kentucky and Tennessee. These oils, 
particularly those known as Pennsylvania grade, are 
rich in paraffines, and contain practically no asphaltic 
hydrocarbons. They are commercially the most valuable 
of the petroleums, on account of the large amount of 
illuminating oils derived from them, but are of little 
value for road purposes, as the base retained after evapora- 
tion of the more volatile constituents is lacking in binding 
properties. 

The Ohio-Indiana oilfield includes the oils of Western 
Ohio and Indiana. They, like- the Pennsylvania oils, are 
of little value for road purposes, but they differ from the 
Pennsylvania oils in containing considerable sulphur and 
in giviag a less amount of illuminating oils. 

The Illinois oilfield covers the oil districts in Illinois 
and a part of Northern Kentucky. These oils vary 
considerably in character, and seem to be of less com- 



BITUMINOUS MACADAM ROADS. 175 

mercial value than the oils of the more eastern fields. 
Some are quite similar to those of the Ohio-Indiana 
field, while others contain considerable percentages of 
the asphaltic hydrocarbons and approach the semi- 
asphaltic oils in character. The recent development 
of this field has been very rapid, and large quantities 
of these oils are now being produced. 

The mid-continent oil field comprises the Kansas and 
Oklahoma oils and those of Northern Texas. These 
oils, like those of IlHnois. are quite varied in character. 
They contain considerable quantities of the asphaltic 
hydrocarbons as well as those of the paraffine series 
and may be classed as semi-asphaltic. Very large 
quantities of petroleum are produced in this district, 
which are sold at lower prices than any of the others. 

The Gulf oil field includes Louisiana and Texas. These 
oils usually contain more asphaltic and less paraffine 
hydrocarbons than the Illinois or Kansas oils. They 
yield residues superior to the others as road materials, 
because of possessing better binding properties. 

The California oil field produces oils composed mainly 
of asphaltic hydrocarbons. These oils vary widely 
in density, the fighter ones being less suitable for road 
purposes, unless refined, on account of the greater per- 
centages of volatile hydrocarbons present. When dis- 
tilled, they yield residues similar in character to asphalt, 
which have been employed for pavements under the 
name of artificial asphalts. 

Crude petroleums are frequently used as dust layers 
on roads, and all classes of oils have been tried for this 
purpose. The parafiine oils are of use for very temporary 
effect only. These oils are also objectionable on account 
of the sticky black mud frequently formed in wet weather. 
Tlie semi-asphaltic oils of the mid-continent and Texas 



176 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

fields are more satisfactory, not possessing the objectionable 
properties of the paraffine oils, but the effect of these 
when crude oil is used is also lacking in permanence and 
does not exert a binding effect upon the road surface. 
The asphaltic petroleums of the California field have 
frequently given good results when used in a crude state, 
binding together the material of the road surface, and 
causing it to gradually harden. The large quantity 
of excellent material available for such use at low cost 
has led to the development of the oiled earth road in 
Cahfornia (discussed in Art. 29) as well as to the extensive 
use of oiled macadam roads. 

Petroleum Residuums. The character of the residue 
obtained by the distillation of petroleum depends upon 
the kind of petroleum used and the extent to which the 
distillation is carried. The residuums may be either 
fluid or solid, according to the extent to which the lighter 
oils have been driven off. 

The fluid residues from the semi-asphaltic petroleums 
are largely used as road oils, under the name of asphalt 
oils. Many of these possess good binding properties, 
and are satisfactory materials for the purpose. These 
oils are also frequently employed as fluxes for the solid 
bitumens in preparing paving mixtures. 

The binding properties of road ofl depend upon the 
presence of the heavy asphaltic hydrocarbons, which 
possess adhesive properties and will remain in the road 
after the evaporation of the lighter hydrocarbons. Oils 
for this purpose therefore should contain a considerable 
percentage of the heavy hydrocarbons (bitumen insoluble 
in 88 degree naphtha) and should have a low percentage 
of paraffine scale. 

The solid residuums from asphaltic or semi-asphaltic 
petroleum are commonly known as oil asphalts, and are 



BITUMINOUS MACAD.\:^1 ROADS. 177 

employed in much the same manner as the natural 
asphalts. These materials vary greatly in character, 
depending upon the care used in preparation as well as 
the petroleum from which they are made. 

Blown oils are semi-solid residuums prepared by blow- 
ing air through a fluid residuum, causing a thickening 
of the oil. The oxygen of the air combines with a part 
of the hydrogen of the hydrocarbons, thus producing a 
change in the characters of the hydrocarbons. 

These oils have been used to considerable extent for 
road purposes when made from semi-asphaltic petroleums, 
and, when carefully prepared, have shown good results. 



Art. 46. Solid Native Bitumens. 

The solid native bitumens which are of interest in 
paving or road work include the asphalts, gilsonites and 
grahamites. These bitumens consist, like the petroleums, 
of natural mixtures of hydrocarbons, but are composed 
mainly of the heavier hydrocarbons and occur as solids. 
Mr. Clifford Richardson divides * the hydrocarbons 
occurring in these bitumens into foar classes which he 
calls: 

Petrolenes, including those hydrocarbons which are 
volatilized at 325 F. in 7 hours. 

Malthenes, including the oils which are soluble in SS° 
Baume naphtha. 

Asphahenes, including the heavier hydrocarbons not 
soluble in naphtha but soluble in cold carbon tetrachloride. 

Carhenes, including hydrocarbons soluble in carbon 
bisulphide, but insoluble in cold carbon" tetrachloride. 

This classification is also frequently extended to the 

* The Modern Asphalt Pavement, New York, 1905. 



1 78 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

hydrocarbons occurring in the petroleums, which are 
very similar in character. 

The solid native bitumens can only be used for road 
purposes by being combined with fluid bitumens to 
bring them to proper consistency. In the use of these 
materials as road binders, petroleum residuum is employed 
as a flux to form asphaltic cement in the same manner 
as in the construction of asphalt pavement (see Art. 67). 

The asphalts consist mainly of malthenes and 
asphaltenes, usually mixed with a certain amount of 
mineral matter, and need to be refined before using to 
remove water and impurities they may contain. They 
are produced to only a limited extent in the United States, 
and while used largely in the construction of asphalt 
pavements, are employed very little for bituminous 
macadam roads on account of the greater cost as com- 
pared with petroleum and tar products. 

Gilsonite differs from asphalt in being more largely 
composed of unsaturated hydrocarbons, and less soluble 
in naphtha. It occurs in Colorado and Utah, and is very 
nearly pure bitumen, containing very little mineral 
matter. It is used to some extent in the construction 
of asphalt pavements, as well as in macadam road con- 
struction for which, when properly prepared, it seems a 
desirable material. 

Grahamite differs from asphalt in being composed 
mainly of asphaltenes and carbenes, as it is but slightly 
soluble in naphtha, and much less soluble in carbon 
tetrachloride, than the other asphaltic materials. It has 
been used in asphalt pavements. 

These materials and the methods of using them arj 
discussed more fully in Chapter IX. 

Rock Asphalt is the name applied to sandstone or 
limestone impregnated with bitumen. These materials 



BITUMINOUS MACADAM ROADS. 1 79 

differ widely in the character of the rock, as well as in 
the kind and quality of the bitumen with which it is 
impregnated. It may vary from sand, in which the 
individual grains are held together only by the bitumen, 
to solid rock in which the pores are filled with bitumen. 
Some of these materials are largely used for asphalt 
pavements (see Chapter IX). They are also used to 
some extent as fillers in bituminous macadam construc- 
tion. Materials of this character are somewhat widely 
distributed over the United States, but on account of 
the wide variation in the material, much of it is not 
suitable to this use. Good results have been obtained 
in the use of Kentucky rock asphalt, consisting of sand- 
stone impregnated with about 6 to 8 per cent of rather 
soft bitumen, which hardens upon exposure to the air, 
through the volatilization of some of the light oils. 

Art. 47. Tar Products. 

The coal tar used in road work is obtained as a by- 
product either in the manufacture of illuminating gas 
or in the burning of coke. Very large quantities of tar 
are produced in the United States, although until quite 
recently, the value of tar has been so small that it has 
been considered a necessary evil, and but little attention 
given to the character of the tar produced. 

Tars are made up of a mixture of hydrocarbons of 
extremely variable character, and it is customary to 
classify these as light oils (volatilizing below 170° C), 
heavy oils (volatilizing between 170° and 270° C), and 
pitch, which is the residue not volatilized. The light 
oils are also frequently divided into those which volatilize 
below 110° C. and those volatilizing between 110° C. 
and 170° C. Crude tars commonly contain some 



l8o A TEXT-BOOK ON ROADS AND PAVEMENTS. 

ammoniacal water, which is included in the distillate at 
iio° C, and is objectionable in tars for road work, if 
present in appreciable quantities. 

The value of tar for road purposes depends upon the 
method and care used in its production, as well as upon 
the character of the coal from which it is obtained. The 
temperature at which the coal is distilled has aa important 
effect upon the quality of the tar, as the oils formed at 
different temperatures differ considerably in character. 

In the manufacture of coal gas, bituminous coal is 
heated in a retort until the more volatile parts are driven 
off, leaving a residue of coke. The gas and tar are sep- 
arated by passing the distillate through water under 
which the tar is condensed, the gas passing on through 
a condenser and other apparatus in which the remainder 
of the tar is removed. The chemical changes which 
occur in the distillation of coal are but little understood. 
They consist in a breaking up of the compounds of which 
the coal is composed and the formation of new combina- 
tions, which depend upon the temperature at which the 
distillation takes place. At a high temperature the 
dissolution is more complete, a greater quantity of gas 
being formed, leaving less of the heavy oils in the tars, 
which contain more solid bitumens and more free carbon 
than those formed at lower temperatures. The tars 
formed at high temperatures are not so desirable for road 
purposes on account of the lack of a sufficient quantity 
of the heavy oils, and because of an excess of free carbon. 

Coke-oven tar is obtained from coal in much the same 
manner as that produced in the manufacture of gas, 
excepting that the chief object is to produce coke instead 
of gas. Large quantities of tar are now obtained from 
this source, but only a comparatively small portioii of 
the coke ovens in use are constructed with a view to 



BITUMINOUS MACADAM ROADS. l8l 

saving the by-products, and in most instances tiie gas 
and tar are allowed to escape and no effort made to save 
them. The great increase in the uses of tar and tar 
products is, however, causing a change in this respect, 
and much greater quantities of tar may reasonably be 
expected in the future from this source. These tars are 
similar to those from gas works, but are usually formed 
at lower temperatures, and thus contain larger per- 
centages of heavy oils and less free carbon. 

Water-gas tars are produced by the decomposition 
of petroleums, or petroleum distillates, in the carburetting 
of water gas. The petroleum oils are broken up into 
light oils, or gases, which impart illuminating value 
to the water gas, and heavier oils, which are condensed 
as tar. These tars are lighter materials than the coal 
tars, containing a larger percentage of heavy oils and 
less of pitch residue. They are usually low in free car- 
bon, do not contain ammonia water, and frequently 
are desirable materials for use as dust layers in road work. 

REFINED TARS 

Tars, like petroleums, are refined by fractional dis- 
tillation, the character of the residual depending upon 
the extent to which the lighter oils have been driven 
off, as well as upon the nature of the original tar. In 
refining, the tar is divided into several fractions by sep- 
arating the distillates between certain temperatures, and 
these fractions are again distilled to separate into desired 
products, such as benzol, naphtha, carbolic acid and 
naphthalene. 

The residue may be liquid or solid according to the 
temperature to which the distillation has been carried, 
and the extent to which the heavy oils have been removed. 



1 82 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

The semi-solid and solid residues are termed pitches 
and are classified according to the temperature required 
to liquefy them. Sufficient heavy oil must be left in 
the residue to render it semi-fluid if it is to be used as a 
road binder. The solid residues are, however, some- 
times used in the same manner as the solid native bitu- 
mens, by fluxing them with other oils. 

In some instances, when the distillation is carried so 
far as to remove the heavy oils, leaving a solid residue, 
parts of the heavy oils are returned to the residuum 
before it cools, thus reducing it to a semi-fluid condition, 
and forming what is known as a cut-hack product. Usually, 
where this method is followed, the naphthalene is removed 
from the heavy oil, which is an advantage to the use of 
the residual for road work. 

DEHYDRATED TARS 

Sometimes when tars are not to be distilled for the 
separation of the products obtained from the oils, they 
are prepared for use in road work by heating sufficiently 
to drive off the water and some of the light oils and are 
then known as dehydrated tars. These tars are superior 
to crude tars, as the presence of water is objectionable 
in tars for use as road binders. This is particularly the 
case with those containing ammoniacal water. 

Art. 48. Tests for Bituminous Materials. 

On account of the wide variation in character of 
bituminous materials which may be available for road 
work, it is very essential that tests be applied for the 
purpose of determining the properties of the various 
bitumens and their suitability for use in construction. 



BITUMINOUS jMACADAjM ROADS. 183 

Considerable variation exists in the tests applied for this 
purpose by various authorities, as well as in the methods 
of conducting them, and but little has been accomplished 
towards standardizing such work. The tests here enu- 
merated are all used to some extent and are of value for 
some materials, but all of them are QOt applicable to any 
one material. 

SPECIFIC GRAVITY. 

The determination of specific gravity is nearly always 
important. The method of determination must of coarse 
depend upon the consistency of the bitumen. As the 
specific gravity of bitumens varies with the temperature, 
it is essential that the determination be made at standard 
temperature, and 25° C. (77° F.) is ordinarily employed 
for the purpose. It is customary to state the specific 
gravity in terms of that of water at the same temperature. 

When the bitumen is quite liquid, a hydrometer gradu- 
ated to read the specific gravity directly is the most 
convenient method and is commonly employed. When 
the materials are too viscous to permit the use of a hydrom- 
eter, it is usual to employ a picnometer. Mr. Hubbard 
describes * a form of picnometer specially suited to this 
work, while Sommer has devisedf a specific gravity 
apparatus intended for semi-solid and solid bitumens 
by suspending a cup containing a definite volume of the 
bitumen from a hydrometer, the stem of which is grad- 
uated to read the specific gravity. 

A knowledge of the specific gravity of a bitumen is 
useful both as assisting in determining the character of 
the bitumen and in indicating the treatment to which it 
may have been subjected. Crude parafiine petroleums 

* Dust Preventives and Road Binders, New York, iqio. 
t Proceedings American Society for Testing Materials, 1909. 



184 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

vary in specific gravity from about 0.75 to 0.85, and is 
usually lower than asphaltic petroleum, which may have 
a specific gravity of about 0.92 to 0.97, the semi-asphaltic 
oils being between the two. Residual oils are heavier 
than crude oils of the same character, the difference 
being to some extent indicative of the extent of distilla- 
tion to which the oil has been subjected. The specific 
gravity of coal tar is largely influenced by the amount of 
free carbon it contains. Refined tars suitable for road 
purposes usually vary in specific gravity from about 
1. 15 to 1.22, the higher value representing a rather large 
percentage of free carbon. 

COMPOSITION OF BITUMINOUS MATERIALS. 

In all examinations of bituminous materials it is nec- 
essary to determine the percentages of bitumen and of 
other organic and inorganic matter in the materials. 
For petroleums and solid native bitumens it is customary 
to determine the quantity of organic matter not classed 
as bitumen, and the percentage of inorganic material 
present. For tars, it is usuahy necessary to determine 
also the percentage of free carbon. 

In testing native bitumens, the light oils are separated 
from the heavy ones by testing the solubility in 86° Be. 
or 88° Be. naphtha, and with asphalts the carbenes are 
separated by the solubility in cold carbon tetrachloride. 
In the examination of tars the light and heavy oils are 
separated by fractional distillation. 

! Total Bitumen. All organic matter which is soluble 
in carbon bisulphide is classed as bitumen. Methods of 
making the test as adopted by the American Society for 
Testing Materials are given in Art. 68. The following 
more rapid method, which has also been recommended 



BITUMINOUS MACADAM ROADS. 185 

by a committee of the same society, is more commonly 
used in testing road materials, 

" From I to 10 grams of the water-free material (de- 
pending upon the amount of bitumen present) is weighed 
into a 150-C.C. Erlenmeyer flask, the tare of which has 
been previously ascertained, and treated with 100 c.c. 
of carbon disulphide. The flask is then loosely corked 
and shaken from time to time until practically all large 
particles of the material have been broken up, when it 
is set aside for not less than 15 hours. At the end of this 
time the contents of the flask are decanted off upon a 
weighed Gooch crucible fitted with long-fiber amphibole 
asbestos filter. The residue remaining in the flask is 
then washed with 50 c.c. of carbon disulphide, allowed 
to settle, and decanted as before, the insoluble matter 
being finally brought upon the filter and washed with 
100 c.c. carbon disulphide, or until the w^ashings are 
practically colorless. The filter and contents are then 
dried at 125° C, cooled, and weighed. Should any 
residue remain in the flask, it is also dried and weighed, 
and this weight added to that of the residue in the 
crucible. The filtrate should be burned off and ignited 
to an ash, and the weight of the ash thus obtained added 
to that of the insoluble residue. The weight of the total 
residue deducted from that of the original material 
gives the weight of bitumen soluble in cold carbon 
disulphide. In case of tars and pitches the percentage 
of insoluble residue, determined as above, minus that 
of any ash which may be found by igniting a separate 
sample, is reported as free carbon." 

Free Carbon. In the examiaation of coal tars, or tar 
products, the determination of free carbon is a matter 
of importance, as this is usually considered an undesirable 
constituent in a road tar, and limits are commonly set 



1 86 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

in specifications to the percentage which may be present. 
A method of determining free carbon is given above 
in the test for total bitumen. 

Fixed Carbon. The term fixed carbon is apphed to 
the residual coke resulting from burning bitumen in a 
closed crucible, and in the absence of oxygen. The 
test is frequently used in the examination of petroleum 
residuums and asphalts to indicate the extent to which 
these materials contain the heavier hydrocarbons of 
asphaltic character. A method of conducting the test 
as recommended by a Committee of the American Society 
of Civil Engineers is as follows : 

'' About I gram of the compound is weighed into a 
platinum crucible i| to ij inches high. The crucible 
with the lid on is heated, first gently, and then until no 
more smoke and flame issues between the crucible and 
the lid. It is then heated 3^- minutes in the full heat of 
the burner; then cooled and weighed. The crucible 
lid is then removed and the crucible and contents allowed 
to remain in the full heat of the burner until the carbon 
is burned off, and then weighed again. The difference 
between these two weights is the fixed carbon. 

Naphtha Soluble Bitumen. Petroleum and asphalt 
bitumens are commonly tested as to their solubility in 
naphtha derived from parafiine petroleum. This test is 
made to determine the relative proportions of the heavier 
hydrocarbons (asphaltenes) , which are insoluble, and of 
the oils (malthenes), which are soluble in the naphtha. 

Naphtha for this purpose is commonly required to have 
a density of 88° Be. and a boiling-point between 40° C. 
and 55° C. Some authorities prefer to use naphtha of 
86° Be. gravity, on account of it being more easily 
obtainable, although it dissolves a little more of the 
bituminous material and is not quite so satisfactory for 



BITUMINOUS MACADAM ROADS. 187 

this reason. In reports of this test, the density of the 
naphtha, and temperatures between which it distills should 
always be given, and results should be stated in terms of 
total bitumen. This test is conducted in the same manner 
as that for solubility in carbon disalphide. 

Distillation Test. This test is made upon tars to 
separate the oils which distill at different temperatures, 
and determine the proportions of each contained by the 
tar. 

The test is made by heating the tar in a retort, and 
collecting the distillate in a condenser pipe, which is 
changed as each temperature is reached in the distilla- 
tion, and the distillate cooled to standard temperature 
and its volume measured or its weight taken. 

Mr. Hubbard recommends * the use of temperatures 
110° C, 170° C, and 270° C, as points of division. 
The fraction which distills over up to 110° C, includes 
the water and ammonia compounds, with certain light 
oils. These separate in the condenser and may be 
separately measured. The distillates between no C. 
and 170° C, are regarded as light oils; those between 
170 C. and 270° C as heavy oils, and the residue as pitch. 

In tars for road binders, water and ammonia compounds 
should not be present in appreciable quantities, but a 
small percentage of light oils is desirable, and if the tar 
is to be used as a binder in construction of bituminous 
macadam, it should contain at least 50 per cent of mate- 
rial not volatilized at 270° C. 

TESTS FOR CONSISTENCY. 

For the purpose of determining the consistency of 
bituminous materials, three tests are commonly employed; 

* Dust Preventives and Road Binders, New York, 1910. 



1 88 A TEST-BOOK ON ROADS AND PAVEMENTS. 

the viscosity test for fluid bitumens, the float test for 
materials too viscous for the viscosity test, and the penetra- 
tion test for solid materials. 

Viscosity Test. Viscosity is measured by determining 
the time required for a given volume of the liquid under 
test to pass through a small opening, and comparing 
with the time required for the same volume of water, 
at standard temperature (25° C), to pass the same 
opening. 

The Engler viscosimeter is commonly employed for 
this purpose. It consists of a covered brass vessel with 
a conical bottom, to which is fitted a vertical outflow 
tube 20 mm. long, with a diameter at top of 2.9 mm. 
and at bottom of 2.8 mm. The tube is closed by a hard- 
wood stopper, which extends through the cover of the 
vessel. The vessel sets in a bath of heavy oil which is 
heated by a ring burner underneath. A measuring 
flask is placed beneath the outflow tube to receive the 
liquid as it passes through the opening. The time 
required by a given volume of the bitumen at the desired 
temperature to pass the opening is measured, and the 
viscosity computed by dividing the time so obtained 
by the time required for the same volume of water at 
25° C. to pass through. 

Float Test. This test is sometimes applied to materials 
too viscous for the Engler viscosimeter. The New York 
Testing Laboratory Float Apparatus * is employed in 
making the test. It consists of a float or saucer, with 
an opening in the bottom, into which a brass collar may 
be screwed. The collar is filled with the bitumen 
(softened by heat), and then placed in ice water at 41° F. 
for 15 minutes. The collar is then screwed into the 
float, which is placed upon the surface of water at 90° F. 

* See Engineering Record, Vol. LIX, 584. 



BITUMINOUS MACADAM ROADS. 189 

When the bitumen softens, water is admitted to the 
saucer and the apparatus sinks. The time in seconds re- 
quired for the apparatus to sink is taken as the consistency. 

Penetration Test. For determining the consistency 
of sohd or semi-sohd bitumens, the penetration into the 
bitumen of a standard needle, under a constant weight, 
is measured. The tests are made at standard tem- 
perature and for definite times. Methods of making 
the test for road bitumens are the same as those employed 
for asphalt paving mixtures, which are described in Art. 69. 

The American Society for Testing Materials has 
recommended * the following as standards for this test : 

" The penetration of bitumen shall be the distance 
expressed in hundredths of a centimeter that a No. 2 
needle will penetrate into it vertically without friction at 
25° C. under a stated weight applied for a stated length 
of time, the factors of weight and time being determined 
as follows: 

*' The material shall lirst be tested for five seconds 
under a weight of 100 grams. If this results is less than 
ten, the penetration shall be determined under a weight 
of 200 grams applied for one minute; if between 10 
and 300, the penetration shall be determined under a 
weight of 100 grams applied for 5 seconds; if greater 
than 300, the penetration shall be determined under a 
weight of 50 grams apphed for 5 seconds. In every 
case the factor of weight and time shall be stated when 
reporting the penetration, and whene\^er possible to 
obtain both readings, the penetration under a 100-gram 
weight for 5 seconds and under the modified weight and 
time shall both be reported. \Vlien testing material 
softer than 100 penetration, a containing receptacle not 
less than i\ inches in diameter shall be used. 

* Proceedings, American Society for^Testing Materials ,Vol. XI, p.247. 



IQO A TEXT-BOOK ON ROADS AND PAVEMENTS. 

"It is recommended that the penetration may be 
determined at o° C. (32° F.) and 46° C. (11.4.8° F.) in 
addition to the 25° C. (77° F.) test." 

EVAPORATION TEST. 

This test is made for the purpose of determining the 
extent to which the material will give off the lighter 
hydrocarbons when heated. The following method of 
making the test is recommended by the American Society 
for Testing Materials: 

" The loss on heating of oil and asphaltic compounds 
shall be determined in the following manner: Twenty 
grams of the water-free material shall be placed in a 
circular tin box with vertical sides, measuriag about 
2 cm. in depth by 6 cm. in diameter, internal measure- 
ment. The penetration of the material to be examiaed 
shall, if possible, be determined at 25° C. and the exact 
weight of the sample ascertained. The sample in the 
tin box shall then be placed in a hot-air oven (New York 
Testing Laboratory oven without fan), heated to 163° C. 
(325° F.) and kept at this temperature for 5 hours. At 
no time shall the temprature of this oven vary more than 
2° C. from 163° C. When the sample is cooled to normal 
temperature, it shall be weighed and the percentage 
of loss by volatilization reported. The penetration of 
the residue shall then, if possible, be determined at 25° C. 
as upon the original material, and the loss in penetration 
found by subtracting this penetration from the penetra- 
tion before heating. In preparing the residue for the 
penetration test it shall first be heated and thoroughly 
stirred while cooling." 

When this test is apphed to solid bitumens, it is intended 
to indicate the possibility of changes taking place in the 



BITUMTXOUS MACADAM ROADS. 191 

character of the bitumen through evaporation, during 
application or subsequent to use in the road. It is com- 
mon, when so using the test, to make penetration test 
of the residue from the evaporation test as well as of the 
original bitumen. 

MELTING POINT. 

The temperatures at which bituminous materials 
become sufhciently soft to flow, are frequently deter- 
mined and are known as the melting-points of the 
materials. 

The following method of making the test is recom- 
mended by a Committee of the American Society of Civil 
Engineers : 

Melting-point of Residue from Evaporation. The mate- 
rial whose melting-point is to be determined, is melted 
and poured into a mold that will make a ^-inch cube. A 
No. 10 gauge wire about 6 inches to 8 inches long is bent 
at right angles for a length of J inch at one end and the 
center of the cube is placed on this end so that one of the 
diagonals of the vertical face of the cube is parallel to the 
long part of the wire. Take a bottle of a size about 2 
inches in diameter and 4 inches high and place a piece 
of white paper in the bottom of it. Pass the long part 
of the wire through the cork of the bottle so that the lower 
edge of the cube will be within i inch of the bottom of 
the bottle. Also put a thermometer through the cork 
so that the bulb is opposite the cube. Place the bottle 
in a water or oil bath and raise the temperature of the 
bath at a rate of 3 to 6° C. a minute. The melting- 
point of the material is the temperature of the thermom- 
eter inside the bottle at the time that the material 
touches the paper in the bottom of the bottle. 



192 A TEXT-BOOK ON ROADS AND PAVEMENTS. 
PARAFFINE TEST. 

For the determination of the amount of parafhne 
scale present in bitumens, the following method is usually 
employed : 

Paraffine. One hundred grams or less of the 'com- 
pound is distilled rapidly in a retort to dry coke. 

Five grams of the well-mixed distillate is treated in a 
2-ounce flask with 25 c.c. Squibbs absolute ether; after 
mixing thoroughly, 25 c.c. Squibbs absolute alcohol is 
added and the flask packed closely in a freezing mixture 
of finely crushed ice and salt for at least 30 minutes. 
Filter the precipitate quickly by means of a suction pump, 
using a No. 575 C. S. & S. 9-cm. hardened filter 
paper. Rinse and wash the flask and precipitate (with 
I to I Squibbs alcohol and ether mixture cooled to —17° 
C.) until free from oil (50 c.c. of wahing solution is 
usually sufficient) . When sucked • dry remove paper, 
transfer waxy precipitate to small glass dish, evaporate 
on steam bath and weigh paraffine remaining on dish. 

Calculation. Weight of paraffine divided by weight of 
distillate taken and multiplied by per cent of total dis- 
tillate used from original sample, equals per cent of 
paraffine. 

Art. 49. Surface Treatment. 

The kinds of bitumen employed and the details of 
application vary considerably in the practice of different 
engineers who construct roads by this method. In all 
cases, it is insisted that, before the application of the 
treatment, the surface of the macadam road shall be in 
smooth, firm condition, free from dust and dirt. Where 
heavy oils, or tars, are being used, dust will prevent 
penetration into the surface of the road, and holes, or 



BITUMINOUS MACADAM ROADS. 193 

depressions, will cause an accumulation of oil, resulting 
in soft spots in the surface. 

The method most largely employed is, after cleaning 
the road surface of dust, to spread a coating of hot oil 
or tar over the surface which is then covered with a layer 
of sand or stone chips to absorb the surplus bitumen and 
rolled to a smooth surface. The tar or oil is frequently 
spread by the use of fiat-nosed watering pots, or ladles, 
or sometimes by the use of hose attached to the kettles. 
The bitumen is then broomed into the surface in order 
to secure a uniform coating. This is accomplished by 
laborers with stiff brooms, who follow the distributors, 
and brush any excess of oil or tar upon uncovered spaces. 

On large work, it is more economical to distribute 
the bitumen from tank wagons, in which, the oil is heated 
and which are arranged with various forms of distributors 
to apply the hot oil to the surface. It is necessary that 
a uniform pressure be maintained on the distributors, 
and a number of devices have been tried for this purpose, 
most of which have not been used to sufficient extent to 
fully demonstrate their value. The most successful 
distributors seem to be those in which the bitumen is 
sprayed upon the road by compressed air, and a number 
of different appliances of this kind are now on the market. 
Some of these, while capable of making a good distribu- 
tion of oil, are liable to quickly get out of order, and in 
some instances it has been found more economical to 
distribute by means of hose attached to the tank wagons. 
For this purpose, a nozzle which atomizes the oil is desir- 
able, the distribution being made under a constant pres- 
sure. 

After the oil has been spread, it is desirable that it 
be allowed several hours for absorption into the road 
surface before applying the layer of chips. Very com- 



194 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

monly, however, the chips are immediately appHed, and 
rolled. Sometimes a second or even a third application 
of bitumen and chips is made to the surface, producing 
a layer of oiled material on top of the macadam surface. 
When such is the case the material used must possess 
sufficient stability not to become soft in wet weather. 

The bitumen used for surface treatment may be petro- 
leum residuum, or tar, so viscous as to require heating 
before application. The heating is commonly done in 
kettles or tanks, mounted on wheels. A temperature 
of about 1 00° C. is usually required, although some- 
times harder bitumens are employed, for which somewhat 
higher temperatures are necessary. 

The quantity of bitumen required for a surface treat- 
ment with heavy oil depends upon the character of the 
road metal, and extent to which the oil may be absorbed, 
as well as upon the method of treatment adopted. The 
quantities used vary from about J to f gallon per square 
yard of surface, and the cost from about 5 cents to 12 
cents per square yard. 

The method of surface treatment used in the construc- 
tion of telford streets in St. Louis by Street Commis- 
sioner Travilla is shown by the following extracts from 
his 191 1 specifications: 

MACADAM OR SECOND COURSE. 

" When the telford foundation has thus been formed, 
there shall be spread a layer of clean, hard limestone 
macadam, free from clay, earth or rubbish; which 
layer, when thoroughly compacted, shall be 4 inches 
in depth. The stone shall be so broken that all will 
pass a 3-inch ring and none will pass a ij-inch ring. 
The stone shall be broken to conform to the above re- 
quirements before being brought on the line of the work. 



BITUMINOUS MACADAM ROADS. 195 

This course shall be thoroughly consolidated by a roller, 
as above specified, and any unevenness in the surface 
shall be corrected before spreading the Hmestone screen- 
ings. The macadam course having been finished, the 
interstices of the stones shall be completely filled with 
clean macadam screenings, containing 50 per cent dust. 
This layer shall then be flooded and rolled until it is 
compact and solid, and ceases to creep under the action 
of the roller. RolHng shall be continued until the 
screenings and water flush to the surface upon all parts 
of the roadway. The surface of the macadam shall be 
broomed immediately after rolling, leaving the clean 
stone projecting; the voids, however, to be thoroughly 
filled, 

ROAD OIL. 

" When the macadam surface, as above prepared, 
has thoroughly dried out to the satisfaction of the street 
commissioner, hot road oil shall be spread over the same 
to the amount of ^ gallon per square yard of surface. 
The oil, as below specified, shall be applied to the road- 
way surface at a temperature of at least 250° F. Proper 
sand barricades shall be constructed along the edge of 
the granitoid gutters to prevent the oil from flowing into 
the same. If the street commissioner deems it neces- 
sary, he may require the macadam surface to be thor- 
oughly hand-broomed and to be lightly sprinkled with 
water before the oil is applied. 

"The specific gravity of the road oil shall not be less 
than 0.959 (16.0° Be. at 60° F.). 

'The loss upon heating a 20-gram sample of the oil 
in a vessel 2 J inches in diameter and ij inches high, 
with vertical sides, for 5 hours at 325° F. (163° C.) 
shall not exceed 3 per cent by weight. It shall not con- 



196 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

tain more than ^ per cent by weight of matter insoluble 
in carbon bisulphide." 

The following method of treating the surfaces of 
finished macadam roads is given "^ by Mr. Charles W. 
Ross, as used at Newton, Mass: 

" Several macadam-surfaced streets, having varying 
grades up to a maximum of 9 per cent, and subjected to 
heavy horse-drawn and automobile traffic, were selected 
for the liquid asphalt treatment. The method used was 
as follows : A quantity of sand was heated to a temperature 
of about 200 ° F., dumped in a pile, and leveled. The 
asphalt was poured over the hot sand in the proportion 
of I gallon to each cubic yard of sand, and then the whole 
mass was turned with shovels or mixed in a concrete 
mixer. (The latter being preferable on account of cost.) 
This work was done at the pit. The mixture was teamed 
to the work and spread on the roadway to a depth of 
less than ^ inch, being raked even with 14-tooth wooden 
rakes. RolKng was not considered necessary and the 
street was kept open for traffic at all times. The cost 
of this treatment was about 3 cents per square yard. 
It has the advantage of leveling and building up the 
surface of the road, each new application providing a 
new wearing surface. This work has remained in 
perfect condition without further expense since the summer 
of 1909." 

Surface treatment of macadam roads with bituminous 
materials has been quite largely used in the United States 
during the past few years. In general, when road sur- 
faces in good condition have been properly treated 
with good materials, the results have been fairly satis- 
factory, particularly when required to resist automobile 
traffic. In some instances, however, these roads do not 

* Transactions, American Society of Civil Engineers, Vol. LXXIII, p. 47. 



BITUMINOUS MACADAM ROADS. 197 

seem so satisfactory when the traffic is mainly horse- 
drawn, which traffic may cut through the oiled layer 
upon the surface and cause the road to rut. 

Art. 50. Penetration Method. 

Roads constructed by the penetration methods by 
different engineers vary considerably in details. In 
most instances, the bottom course is constructed in the 
same manner as for water-bound macadam, the voids 
being completely filled with sand or screenings, and rolled 
to a hard surface. After completing the foundation 
course, the second course of road metal is placed. This 
may consist, when hard rock is employed, of stone vary- 
ing in size from about J inch to ij inches, or, when 
limestone is used, from about i inch to 2 J inches in 
diameter, the road metal is placed to the proper depth 
(usually 2^ or 3 inches) aad rolled to an even surface. 
The hot bitumen is then poured over the surface, and 
allowed to run into and coat the stone composing the 
surface course. In some instances a light coating of 
stone chips is used, to partially fill the voids, before the 
bitumen is applied. The bitumen must be quite fluid 
at the temperature of application, in order to run into 
the surface material and thoroughly coat the stone. 
About I J gallons of bitumen per square yard are usually 
required. The surface is next covered with a coating 
of stone chips, which are rolled into the surface. Upon 
completion of the rolling, any surplus fine material 
should be swept from the surface before adding the 
paint coat, which consists in pouring about J or J gallon 
of bitumen per square yard over the surface, covering 
with screenings, and rolKng to a finished surface. 

The bitumen may be applied to the surface by the same 



198 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

methods as are used in surface treatments, although 
some of the automatic distributors do not work so well 
over these comparatively rough surfaces as on the fin- 
ished macadam. 

The method of construction used by the Illinois High- 
way Commission is shown by the following extract from 
their 191 1 specifications: 

SPREADING SECOND COURSE OF STONE. 

" The broken stone for the second course is not to be 
spread before the first course has been completed and 
shoulders made as herein specified. The second course 
shall be of 2i-inch stone and shall be spread to compact 
under rolling to the thickness shown on the plans. 

HARROWING SECOND COURSE OF STONE. 

" After the second course of stone has been spread, 
it shall be harrowed as hereinbefore described for the 
first course of stone. (See p. 148). 

ROLLING SECOND COURSE OF STONE. 

" After the broken stone for the second course has 
been spread and harrowed to the required thickness and 
has a proper cross-section, it is to be rolled with a steam 
roller, weighing not less than 10 tons, until it is compacted 
to form a firm, smooth surface. The rolling is to begin 
at the sides, the shoulders for a width of at least 5 feet 
first being rolled until firm. When completed, the sur- 
face of the shoulders and of the second course of broken 
stone must be smooth and continuous with a cross slope 
as shown on the plans. 



BITUMINOUS MACAD.\M ROADS. IQQ 

UNEVENNESS OR DEPRESSIONS. 

" If any unevenness or depressions appear during 
or after the rolling of the second course, either on the 
surface of the shoulder or the broken stone, suitable 
material shall be added to remove all such unevenness or 
depressions, earth being used on the shoulders and stone 
for the macadam. 

SPREADING CHIPS. 

"After the second course of stone has been rolled and 
completed as specified, the surface voids are to be filled 
with chips, free from dust, which shall be whipped into 
the surface from shovels, the quantity being such as will 
just fill these surface voids. After the chips have been 
whipped into the surface, it shall be gone over with a 
stiff brush broom and all chips remaining on the surface 
of the stone swept into the voids in the surface, and if an 
excess remains after the voids have been filled, they shall 
be swept off to the edge of the macadam. 

SPREADING BITUMINOUS BINDER. 

" Upon the surface prepared as above described, 
there shall be uniformly distributed i gallon per square 
yard of surface of the asphalt binder herein specified. 
The asphalt binder shall be apphed at a temperature 
not less than 350° F., and shall be spread on the surface 
in a manner which will insure that a uniform amount is 
applied to all parts of it. 

ROLLING BINDER. 

" Immediately after the first course of binder is spread, 
the surface is to be rolled, preferably with a tandem 



200 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

roller, weighing not less than 8 tons. The roller must 
be provided with means to keep the surface of the wheels 
sprinkled with water. The rolUng is to continue until 
the surface has become hard, smooth and as closely com- 
pacted as possible. 

SPREADING SECOND COURSE OF CHIPS. 

" After the first course of binder has been spread and 
rolled as above specified, there shall be spread a quantity 
of chips which will be just sufficient to fill the remaining 
voids in the surface. The chips shall be brushed into the 
voids with a stift brush broom. 

SPREADING PAINT COAT OF ASPHALT BINDER. 

" After the second course of chips has been spread 
and brushed into the surface, a second course of the 
approved binder shall be spread at the rate of J gallon 
per square yard. The binder shall be spread at a tem- 
perature of not less than 350° F. and shall be applied in 
such a manner as will insure that a thin even coat of the 
binder covers the entire surface. 

DUSTING AND ROLLING FINISHED SURFACE. 

" After the paint coat of die bituminous compound 
has been spread, the surface shall be dusted lightly with 
coarse quartz sand, not to exceed one (i) cubic yard 
per 300 square yards of surface, and the surface rolled 
with a 10- ton roller, the wheels being wet to prevent 
sticking. After the surface has been rolled, it shall 
be allowed to stand for one-half day before being opened 
to traffic." 



BITUMINOUS MACAD.UI ROADS. 20I 

The method recommended by the Association for 
Standardizing Paving Specifications is shown by the 
following extract from proposed specifications: 

" Upon the bottom course shall be evenly spread 
crusher run stone which shall pass a 3 -inch ring and 
be retained upon a i-inch screen, to a finished depth 
of two and one-half (2J) inches. This course shall be 
dry rolled with a steam roller herein before mentioned 
only until the individual fragments have keyed together, 
the surface, while even and conforming to the required 
crown, being left open or porous in order to allow the 
penetration of the hot bituminous binder. 

" The binder shall be heated in an approved heater 
equipped with a fixed or portable thermometer which 
will clearly and accurately indicate the temperature of 
the binder. The bituminous binder shall be heated to a 
temperature of not less than 250° F., nor more than 350° 
F., and shall be uniformly distributed over the macadam 
by suitable appliances at a rate of not less than one and 
seven-tenths (1.7) gallons to the square yard. Directly 
after appHcation, clean trap rock, or equally satisfactory 
stone chips, free from dust a ad consisting of fragments 
which will pass a i-inch ring but be retained upon a 
three-eighths {^) inch screen, shall be spread over the 
surface in sufficient quantities to fill the surface voids 
and prevent the binder from sticking to the wheels 
of the roller. Care shall be exercised not to apply more 
stone chips than will just fill the interstices and any 
surplus material shall be swept from the surface, as 
directed. The road shall then be rolled until solid, 
more stone chips or screenings being applied as required 
in order to maintain satisfactory conditions. 

" A seal, flush, paint or squeegee coat of the hot 
binder shall be uniformly distributed over the surface at 



202 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

a rate of at least one-half (J) gallon to the square yard. 
Clean stone chips or screenings such as previously 
described shall then be spread over this seal coat in just 
sufficient quantity to take up all excess of biader and 
form a smooth well-bonded surface when rolled. The 
road shall be rolled until smooth and firm and to the 
proper lines and grades. 

" The stone must be dry and free from dirt or dust at 
the time of applying the bituminous binder. The applica- 
tion of binder shall not be made when the atmospheric 
temperature is below 50° F. unless specially permitted 
by the engineer." 

Art. 51. Mixing Method. 

The construction of bituminous macadam by the 
mixing method differs from, that by the penetration 
method, in that the bitumen is mixed with the aggregate 
before it is placed upon the road. The lower course 
is formed in the usual manner, and finished as in water- 
bound macadam. The hot bitumen is mixed with broken 
stone, which is also usually heated, thus permitting the 
use of bitumen of harder consistency than could be used 
for pouring or for mixing with cold stone. The quantity 
of bitumen used is commonly somewhat less than with 
the penetration method, and should be sufficient to 
thoroughly coat all the stones of the aggregate. 

It is desirable that the stone be so graded as to reduce 
the voids to a minimum, and sometimes different sizes 
of rock are mixed for this reason. Usually, however, 
in macadam work, careful grading of sizes would be too 
expensive, and the material is used between certain limits 
of size as it comes from the crusher, as in the other methods. 
In the construction of city pavements by the Bitulithic 
process, careful grading of stone into several sizes, and 



BITUMINOUS MACADAjM ROADS. 203 

exact mixing to secure best results, is practiced. This 
will be considered in Art. 73. 

In constructing roads by this method, the mixing is 
done either in mechanical mixers or by hand. On large 
work, mechanical mixing is usually cheaper and more 
satisfactory. A number of portable mixers are now on 
the market, several of which are capable of doing excellent 
work. In hand mixing, the cost depends upon whether 
the plant is arranged for coQvenient handling of the 
materials, with the least amount of labor. The stone, 
in hand mixing, is commonly used cold, or it may be 
heated sufficiently to prevent the sudden chilling of 
the bitumen when it comes in contact with the stone. 
The bitumen employed should be a heavy viscous mate- 
rial, sufficiently soft to flow slightly at the temperature 
of the stone. 

WTien the mixing of the bitumen and stone is com- 
pleted, the concrete is placed upon the road to the desired 
thickness and rolled. Sometimes a layer of stone chips 
is added before rolling to prevent sticking. A paint 
coat is then usually applied to the surface, and covered 
with a light coating of screenings, and the whole rolled 
to a finished surface. 

Bituminous macadam, constructed by the mixing 
method, has been extensively used in England, where it 
seems to be commonly preferred to the penetration, or 
tar grouting, method. The material used in England 
is almost exclusively tar. In many instances, tar concrete 
is prepared at a ceatral plant, located conveniently to 
the supply of aggregate, and shipped ready for use to the 
site of the work. A complete discussion of English 
practice in the construction of tar macadam roads is 
given by Mr. J. Walker Smith.* 

* Dustlcss Roads, Tar ]\Iacadam, London, 1909. 



204 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Art. 52. Selection of Bituminous Materials. 

The selection of bitumens for use on macadam roads 
always depend largely upon local conditions. The kinds 
of material which are most readily available; the char- 
acter of the road metal to be employed and method of 
construction to be adopted; the local conditions of climate 
and traffic which the road is to meet, are all to be con- 
sidered before a proper selection can be made. 

The varying results which have been obtained with the 
same materials in different localities, and under differing 
conditions, make it impracticable, at present, to establish 
any standard specifications for such materials. Expe- 
rience in the use of these materials is, however, rapidly 
shaping practice, and determining the most suitable 
materials for localities in which bituminous macadam 
construction is being used. 

The Illinois Highway Commission uses the penetra- 
tion method in constructing bituminous macadam, 
applying the bituminous binder at a temperature of about 
350° F. The bitumens used are petroleum residuums, 
or oil asphalts, and the 191 1 specifications are as follows: 

ASPHALT BINDER. 

" The asphalt binder used shall conform to the follow- 
ing specifications. The various properties herein de- 
scribed to be determined by the methods proposed by the 
American Society for Testing Materials. 

^'Specific Gravity. The asphalt shall have a specific 
gravity not less than 0.97. 

"Total Bitumen. The asphalt shall be soluble in cold 
carbon bisulphide to the extent of at least 98 per cent. 

"Naphtha Insoluble Bitumen. Of the total bitumen not 



BITUMINOUS MACADAiSI ROADS. 205 

less than 20 per cent nor more than 25 per cent shall 
be insoluble in 86° Be. naphtha. 

^^Loss on Evaporation. When 20 grams (in a tin dish 
2J inches in diameter, with vertical sides) are maintained 
at a temperature of 170° C. for 5 hours in a N. Y. testing 
laboratory oven, the evaporation loss shall not exceed 
2 per cent and the penetration shall not have been de- 
creased more than 25 per cent. 

^^ Fixed Carbon. The fixed carbon shall not exceed 12 
per cent by weight. 

^^Penetration. The penetration as determined with the 
Dow machine using a No. 2 needle, 100-gram weight, 
5 sec. time and a temperature of 25° C, shall not be less 
than 5.0 mm. nor more than lo.o mm. 

^'Paraffi^ie. The asphalt shall not contain to exceed 
2 per cent by weight of paraffine." 

In the construction of bituminous macadam by the 
penetration method in Ohio, by Mr. James R. Marker, 
State Highway Commissioner, the bitumen is applied 
at temperature between 250° F. and 350° F. Several 
kinds of bitumen may be employed, each of which is 
provided for in the specifications, which are as follows: 

OIL ASPHALT. 

" I. The oil asphalt shall have a specific gravity of 
not less than c 965 at 25° C. 

" 2. It shall be soluble in c.p. carbon disulphide at 
air temperature to at least 99.5 per cent and shall con- 
tain not over 0.3 per cent organic matter insoluble. 

" 3. It shall contain not less than 18 per cent nor more 
than 25 per cent of bitumen insoluble in 86° Be. paraffine 
naphtha at air temperature. 

"' 4. WTien tested for 5 seconds at 25° C. with a standard 



2o6 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

No. 2 needle weighted with loo grams, it shall show a 
penetration of not less than lo mm. nor greater than 20 
mm. 

"5. When 20 grams of the material is heated for 5 
hours in a cylindrical dish approximately 2 J inches in 
diameter by f inch high, at a constant temperature 
of 163° C, the loss in weight by volatilization shall not 
exceed 3 per cent. The residue remaining shall show a 
penetration of not less than 6 mm. when tested in the 
manner hereiabefore described. 

" 6. Its fixed carbon shall be not less than 7.5 per cent. 

'* 7. The oil asphalt upon delivery shall be free from 
water. 

FLUXED NATIVE ASPHALT. 

" I. The fluxed asphalt shall have a specific gravity 
of not less than i.o.i at a temperature of 25° C. 

" 2. Its solubility at air temperature in c.p. carbon 
disulphide for the following named materials, or materials 
similar thereto, shall be at least 95 per cent for Bermudez 
products, 81 per cent for Cuban products and 66 per 
cent for Trinidad products. 

3. It shall contain not less than 18 per cent nor more 
than 25 per cent of bitumen, insoluble in 86° Be. 
paraffine naphtha. 

" 4. It shall yield not less than 9 per cent nor more 
than 13 per cent of fixed carbon. 

"5. The penetration shall be between 10 and 20 mm. 
when tested for 5 seconds at 25° C. with a No. 2 needle 
weighted with 100 grams. 

" 6. When 20 grams of the material is heated for 5 
ho-irs in a cylindrical dish approximately 2^ inches in 
diameter by f inch high at a constant temperature of 
163° C, the loss in weight shall not exceed 5 per cent. 



BITUMINOUS IMACADAAI ROADS. 207 

The residue thus obtained shall show a penetration of 
not more than 12 mm., nor less than 5 mm. when tested 
in the manner hereinbefore described. 

" 7. The fluxed asphalt upon delivery shall be free 
from water. 

REFINED COAL TAR. 

" I. The tar shall have a specific gravity of not less 
thaa 1. 1 70 nor greater than 1.250 at 25° C. 

"2. On extraction wdth carbon disulphide, it shall 
contain not more than 20 per cent free carbon. 

''3. Upon ignition it shall show not over 0.5 per cent 
inorganic residue. 

'' 4. When the sample of tar is submitted to the float 
test,"^ as hereinafter described, the float shall sink in water 
maintained at 50° C. in not less than 2^ minutes nor more 
than 3 minutes. 

" 5. WTien 250 c.c. of the tar is distilled in a 750-c.c. 
glass retort at a rate not exceeding 2 drops of distillate 
per second, the total distillate to 170°, as registered by a 
thermometer whose bulb is level with the bottom of the 
outlet of the body of the retort, shall not exceed 2 per 
cent by volume of the original material. The total dis- 
tillate to 270° C. shall in no case exceed 50 per cent and 
when the tar contains more than 10 per cent free carbon, 
this distillate shall not exceed 40 per cent by volume of 
the original material. 

" 6. The tar upon delivery shall be free from water. 

REFINED WATER-GAS TAR. 

" I. The tar shall have a specific gravity of not less 
than 1. 160 nor more than 1.185 at 25° C. 

"2. It shall be soluble in c.p. carbon disulphide at 
air temperature to at least 95 per cent. 

* See footnote on p. 208. 



2o8 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

" 3. Upon ignition it shall show not over 0.5 per cent 
inorganic residue. 

" 4. When the sample of tar is subjected to the float 
test,* as hereiaafter described, the float shall sink in water 
maintained at 32° C. in not less than 13 minutes nor 
more than 18 minutes. 

" 5. Whea 250 c.c. of the tar is distilled in a 750 c.c. 
glass retort at a rate not exceeding 2 drops of distillate 
per second, the total distillate to 170° C, as registered 
by a thermometer whose bulb is level with the bottom 
of the outlet of the body of the retort, shall not exceed 
4 per cent by volume of the original material. The 
total distillate to 270° C. shall not exceed 35 per cent 
by volume of the original material. 

" 6. The tar upon delivery shall be free from water." 

* The apparatus used in making the float test is manufactured by 
Howard & Morse, Brooklyn, N. Y., and consists of two parts, an 
aluminum float or saucer, and a conical brass collar. The float contains 
an aperture in the bottom threaded so as to receive the smaller end of 
the brass collar. In using this apparatus, the collar is placed upon a 
brass plate having an amalgamated surface. The collar is then filled 
with the bitumen, after it has been softened by gentle heating. As soon 
as the bitumen has cooled sufficiently to handle, it is placed in ice-water 
for fifteen minutes. It must then be attached to the float, which should 
be immediately placed upon the surface of the water, which is main- 
tained at 50° C. for the coal-tar test, and at 32' C. for the water-gas tar 
test. As the plug of bitumen in the brass collar becomes warm and 
fluid, it is gradually forced out of the coflar and the water gains entrance 
to the saucer. The time in seconds elapsing between placing the appara- 
tus on the water and when the water gains entrance into the saucer 
should be determined by means of a stop-watch. 



CHAPTER VII. 

FOUNDATIONS FOR PAVEMENTS. 
Art. 53. Preparation of Road-bed. 

In forming a road-bed upon which to place a pave- 
ment, the earth should be brought at subgrade to the 
form of a finished road-surface, leaving room for the 
superstructure of uniform thickness to be placed upon 
it. Thorough drainage must of course be carefuUj^ at- 
tended to when necessary. This has been already dis- 
cussed in Chapter II. 

In the construction of a road-bed to support a 
pavement, the same principles are involved as in the 
earthwork of a common road, which has been discussed 
in Art. 26, and the same methods may be employed 
in handling the earth. In grading, the surface should 
be left high enough to allow for the compression pro- 
duced in rolling. The amount of settling to be ex- 
pected under the roller will vary with the character 
of the material and the weight of the roller. With a 
heavy steam roller, the compression may vary from J 
inch for stiff soil in drj^ condition to about 3 inches for 
light porous soil. The allowance to be made can only 
be judged from experience with the soil in question. 

The road-bed, after being brought to the proper 
grade, should be thoroughly compacted by rolling 
before placing the pavement. Sometimes in the use of 
a heavy roller, when the material is of a light nature, it 
is shoved forward in a wave before the roller and re- 

209 



2IO A TEXT-BOOK ON ROADS AND PAVEMENTS. 

fuses to become compacted, in which case a thin layer 
of gravel or small stone placed upon the surface of 
earth before rolling may have the effect of consolidat- 
ing the road-bed under the roller to a hard surface. 

The roller should pass several times over the road- 
bed. When low places are developed, which roll down 
below grade, they should be filled and rolled again 
until brought to proper grade. Passing the roller trans- 
versely over recently filled trenches will always produce 
depressions which require refilling. Where such trenches 
exist, the rolling should be very carefully done. 

In rolling, soft spots are sometimes discovered, which 
cannot be compacted by rolling. In such cases the 
soft material should be removed and replaced with 
better material to a sufficient depth to admit of roll- 
ing the road-bed to a compact surface. 

In some instances, repeated rolling of light material 
with a heavy roller may have the effect of working the 
material loose so that it moves in a wave before the 
roller, although the first rolling leaves the road-bed 
compact. In such cases it is desirable to avoid too 
much rolling. 

Where much grading is to be done, it is usually de- 
sirable to do the rough work before setting the curb 
upon the street, if a new curb is to be placed. It is, 
however, much easier to finish the grade after the curb 
is set, as a line across the street at the top of the curb 
is a convenient means of getting the elevation of points 
on the subgrade. 

Art. 54. Trenches in Streets. 

The opening of trenches for water, gas, and sewer 
pipes in the streets is perhaps the greatest cause of de- 



FOUNDATIONS FOR PAVEMENTS. 211 

struction of pavements to be found in the average 
city. This is especially true of the smaller cities, 
where wear from traffic is not excessive. 

In constructing a pavement in an unpaved street 
an effort should always be made to lay all pipes which 
are likely to be needed in the street for a considerable 
period, in so far as they can be foreseen, before placing 
the pavement. Where a cut is made through a pave«- 
ment for a trench it is a matter of considerable 
difficulty to backfill the trench and replace the pave- 
ment in as good condition as before it was cut, and 
great care is required to prevent the subsequent set- 
tlement of the pavement over the trench. The filling 
of trenches over which a pavement is to be placed 
requires very close inspection, and frequently, neglect 
of such inspection causes much trouble subse- 
quently. 

The most common method of filling trenches in 
unpaved streets is to throw the earth in loosely, and 
pile the surplus earth in a ridge over the trench, leaving 
it for the natural settlement, when w^et w^eather comes, 
to ultimately compact the earth in the trench. 
Usuall}^ the settlement of such a trench will extend 
over a long period, and there is danger of injury to a 
pavement built over the trench, even after several 
months have elapsed and settlement seems to have 
taken place. Rolling will compact the earth in the 
top of the trench, but its effect does not reach to any 
considerable depth in the trench, or prevent later 
settlement. There are many instances in which 
disastrous settlements of pavements have occurred 
over trenches, although the material in the trenches 
had been considerably settled by rains and the surface 
rolled with a heavy roller. 



212 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Flooding. It is common practice to settle the earth 
in trenches by flooding with water. This is accom- 
plished either by repeatedly filling a few inches of 
earth into the trench and then saturating with water, 
or by flooding the trench with a few inches of water 
and filling the earth into the water. It is difficult to 
compact the earth by flooding so that no further settle- 
ment will take place, and it is necessary to use care that 
the earth be not thrown in in too large quantities at 
once, as when the trench is filled with scrapers or 
graders. When the soil is clay, subsequent settlement 
will take place as the clay shrinks upon drying out. 

In filling sewer trenches in this manner there is 
usually danger of breaking the joints of the sewer in 
flooding the trench. Several instances have been noted 
in which this has occurred, and the practice should 
be avoided. 

Tamping. The only method of effectively compact- 
ing ordinary earth in a trench so that no danger of 
subsequent settlement shall exist is by placing the 
earth in thin layers, not more than 4 or 5 inches 
thick, and tamping each layer thoroughly. To accom- 
plish this the earth must be damp enough to pack well, 
but not too wet. 

The earth compacts into smaller space when rammed 
in the trench than it formerly occupied, so that when 
the pipe is small as compared with the size of the 
trench there may not be enough earth removed in 
excavating the trench to entirely refill it. 

Art. 55. Purpose of Foundation. 

The chief object of the foundation or base of a 
pavement is to distribute the concentrated loads which 



FOUNDATIONS FOR PAVEMENTS. 213 

come upon the surface of the road over a greater area 
of the usually softer and weaker road-bed, in order 
that these loads may not produce indentations in the 
surface. 

In a foundation composed of independent blocks 
extending through its thickness, as in the case of a 
stone-block pavement in which the blocks rest directly 
upon the road-bed or upon a thin layer of sand, the load 
which comes upon the top of any block will be dis- 
tributed over the area covered by the base of the block. 

Where the foundation is composed of small independ- 
ent particles, like sand or loose rounded gravel, with 
no cohesion through the mass, the pressure is distrib- 
uted over the base of a cone whose vertex is in the 
point of application of the load, and the inclination of 
whose elements depends upon the friction of the par- 
ticles of the material upon each other. In this case 
the area over which the load is distributed varies 
directly as the square of the thickness of the founda- 
tion. Sand, it is to be obser^^ed, has also the property, 
when confined as in a foundation, on account of its 
incompressible nature, of adjusting itself to a uniform 
pressure and resisting the deformation of the road-bed. 
If the small pieces composing the foundation are 
cemented together, or held as in masses of angular 
fragments by the interlocking of the angles, the foun- 
dation may act more or less as a whole, causing a distri- 
bution of the load over a considerable area, the extent 
of which will depend upon the resistance of the mass 
to bending. 

The bases most commonly employed for pavements 
are sand, broken stone, and concrete. Foundations of 
brick and wood are also sometimes employed for pave* 
ments of the same materials. 



214 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Art. 56, Bases of Gravel and Broken Stone. 

For light service on a good road-bed a satisfactory 
foundation may frequently be constructed of gravel 
or broken stone at much less expense than would be 
required for a concrete base. A foundation of this 
character should be constructed in about the same 
manner as a broken-stone road, the material being 
spread over the road-bed and thoroughly rolled to 
the required form. The stone or gravel employed 
should contain sufficient small material to fill the 
voids in the aggregate, or a binding material may be 
added to aid in compacting the foundation, and to 
close the interstices so as to prevent any settling of 
the material which is used to support the paving 
surface. 

These foundations are sometimes used under brick 
pavements of light traffic with good results. It is 
important, in the construction of brick pavements in 
this manner, that the interstices in the base be thor- 
oughly filled, as otherwise the sand cushion under the 
bricks may gradually settle into the foundation. 

A foundation of this kind can never have the strength 
and permanence of a concrete base, and, while they 
may give good results when well constructed under 
proper conditions, they have frequently been used 
where a small additional expense for concrete would 
have been much more economical in the end. 

Art. 57. Concrete Bases. 

The best base for general use under pavements is 
without doubt that formed of h3^draulic cement con- 
crete. A bed of concrete made of good hydraulic 



FOUNDATIONS FOR PAVEMENTS. 215 

cement, well rammed and allowed to set and harden, 
becomes a practicall}^ monolithic structure, nearly im- 
pervious to water and possessing a high degree of 
strength against crushing. 

The concrete is formed of a mixture of cement, 
sand, and broken stone or gravel. The proportions 
vary for different work and with the character of the 
materials. With good Portland cement the most 
common proportions for ordinary work are about one 
part cement, 3 parts sand,, and 5 to 7 parts broken 
stone. With the various natural cements the pro- 
portions vary somewhat, but are usually about I part 
cement, 2 parts sand, and 4 or 5 parts of stone or 
gravel. 

Natural cement is often employed for this pur- 
pose as being cheaper and possessing ample strength 
for the work, and concrete of the ordinary propor- 
tions with natural cement is to be preferred to that 
made with meager proportions of Portland cement 
giving about the same strength and cost. Proper tests 
should always be imposed for the purpose of securing 
good cement.''' 

Sand for use in mortar should be as clean and as 
free from loam, mud, or organic matter as possible. 
In general the presence of any foreign matter is to be 
avoided. Coarse sand is usually preferable to that 
which is ver\^ fine, provided it be fine enough to give a 
smooth mortar, as it affords better strength. The use 
of a mixture of grains of various sizes is usually desirable 
as giving less voids to be filled by the cement. 

The aggregate used for concrete should be as hard 

* For discussion of tests of cement see "Hydraulic Cement, its Prop- 
erties, Testing and Use," by F. P» Spalding. John Wiley and Sons, 
New York. 



2l6 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

and durable as possible, and that of angular form is 
preferable to rounded. The materials should be uni- 
form in quality. When gravel is used which varies in 
quality, it should be blended by mixing in order to 
obtain uniform strength in the concrete. The best 
concrete will usually be made from the stone contain- 
ing the smallest percentage of voids, provided the 
material be uniform. In a mass of ordinary broken 
stone the voids are usually from 40 per cent to 55 per 
cent of the volume. This may be considerably reduced 
by careful adjustment of the sizes. The broken stone is 
commonly limited in size to 2 or 2 J inches, and the 
whole output of the crusher is used, with the dust 
screened out. The quantity of sand needed is such as 
will fill the voids in the aggregate. 

In preparing the concrete, the cement and sand 
should first be thoroughly mixed while dry, then the 
proper quantity of water be added all at once, and 
the mortar be vigorously worked with hoe or shovel 
for 2 or 3 minutes, until it comes to a smooth and uni- 
form condition. 

The quantity of water should be such as under 
energetic working will reduce the mortar to a soft, 
plastic condition, and should be determined by meas- 
ure. The application of the water from a hose during 
the mixing is objectionable on account of the diffi- 
culty of regulating the quantity to produce mortar of 
proper consistency, 

When the niixing of the mortar is complete, the 
stone or gravel may be added, and the whole mass 
turned several times with shovels until the mortar is 
evenly distributed through the aggregate. The stone 
should be wet by sprinkling before it is mixed with 
the mortar, in order to clean the surfaces of dust and 



FOUNDATIONS FOR PAVEMPZNTS. 21 7 

to prevent the absorption of water from the mortar 
before it sets. 

The concrete, when ready, is placed in position and 
tamped to surface. For this use it is preferable that 
the concrete be of jelly-like consistency, such that it 
will quake under light ramming. The rammer com- 
monly employed consists of a block of wood, or of cast 
iron, 6 to 8 inches square, flat on the bottom, and 
weighing 20 to 30 pounds. The tamping should cause 
the mortar to flush to the surface. 

After completion the foundation should be allowed 
to stand several days before the pavement is placed 
upon it, — 3 to 6 days are usually required, — in order 
that the mortar may become entirely set. During 
setting the concrete should be protected from the 
drying action of the sun and wind, and should be kept 
damp to prevent the formation of drying cracks. 

The quantity of material necessary to make a cubic 
yard of concrete varies with the density of the broken 
stone. For materials measured loose, to make a cubic 
yard of i, 2, 4 concrete will require li to \\ barrels of 
natural cement, y^ to t% cubic yard of sand, and 1% to 
I cubic yard of broken stone. To make i cubic yard of 
I, 3, 6 concrete requires 1% to I barrel of Portland 
cement, -^^ to 1% cubic yard of sand, and y% to I cubic 
yard of broken stone. 

Art. 58. Bituminous Foundations. 

Foundations of bituminous concrete are frequently 
used under asphalt and bitulithic pavements, and, in 
some instances, under other surfaces. These founda- 
tions are constructed in much the same way as bitu- 
minous macadam roads (see Chap. VI). In some 



2l8 A TEXT-BOOK ON ROADS AND PAVEMENTS- 

mstances the bases are formed of concrete composed 
of broken stone and tar, or asphalt cement mixed m 
the same manner as the binder course for an asphalt 
pavement (see Art. 70), and rolling or tamping the 
concrete into placCo 

The more common method of construction is by 
spreading and rolling the broken stone, 4 or 6 inches 
thick, as for a macadam road, and covering the surface 
with a coating of bituminous cement. Coal-tar cement 
is ordinarily used for this purpose, or a mixture of coal- 
tar and asphalt. 

The bituminous foundation is commonly employed 
in the construction of bitulithic pavements (see Art. 73). 
The advantage claimed for it is that it permits the over- 
lying courses to bind into the foundation and holds 
the surface layer in place. Foundations of this kind 
give good results when the road-bed is firm, so that it 
m.ay be rolled solid and is not likely to become unstable. 
It has not, however, the stability of hydraulic cement 
concrete "and should not be used where strength is 
needed or where the road-bed is composed of spongy 
clay or other material which cannot be rolled to pro 
vide a solid sub-foundation. 

Art. 59 Miscellaneous Foundations. 

Brick Foundations. Foundations of brick have fre^ 
quently been used under brick pavements. The pave 
ment in such cases consists of two layers of brick, with 
sand between, and is known as double-layer pavement. 
These foundations are usually formed by placing upon 
the road-bed a layer of sand or gravel 3 or 4 inches thick, 
which is rolled thoroughly to a uniform surface, and 
then receives a layer of brick, commonly laid fiat and 



FOUNDATIONS FOR PAVEMENTS. 219 

with the greatest dimension lengthwise of the street, 
These bricks are laid as closely as possible with 
broken joints. The joints are filled with sand care- 
fully swept in, and the bricks are rammed to a firm 
bearing. 

Upon this course of brick is placed a cushion layer 
of sand, and then the surface laj^er. The bricks of the 
lower layer ma3^ be of a cheaper grade than the sur- 
face paving brick, as they are not required to resist the 
attrition of travel. 

Care must be used to thoroughly fill the joints in the 
foundation layer of brick in order that the sand in the 
cushion layer may not work downward and allow 
the surface bricks to settle. 

These foundations were formerly quite extensively 
used for brick pavements, but have for the most part 
been superseded by concrete or macadam bases. They 
have, in many instances, given good results in use 
when resting upon a firm road bed, but lack the 
strength of the concrete foundation and are not usually 
economical. 

Sand and Plank Foundation, Under many wood 
pavements, and sometimes under brick surfaces, foun- 
dations formed of sand and planks have been used. 
These foundations differ somewhat in construction in 
various localities, but are essentially a bed of sand or 
gravel, upon which is placed a layer of tarred boards 
which support the surface layer. 

It is common to use a la3?^er of sand 3 or 4 inches 
thick, which is compacted by rolling, after which the 
boards are laid lengthwise of the street close together, 
so as to form a floor upon which the blocks may be 
seto With a brick surface a cushion coat of sand is 
used under the surface layer. 



220 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Sometimes two layers of one-inch tarred boards are 
emploj^ed, the lower being laid crosswise of the street 
and the upper lengthwise of it. In other cases the 
boards of a single thickness are nailed to scantling laid 
across the street and bedded in the sand. The boards 
must in all cases press evenly upon the layer of sand 
that covers the road-bed. 

These foundations were used under the round block- 
wood pavement, at one time quite extensively. They 
are employed only where low cost of construction is 
necessary, and are not economical when a durable road- 
surface is to be constructed. 

Sand Foundations. Brick pavements have frequently 
been constructed with onlj^ a cushion coat of sand upon 
the earth road-bed. In some instances, vv^here the 
road-bed is firm and well drained, forming a natural 
foundation, this method of construction has been 
successful under light trafhc, but the failures have been 
numerous, and it is only under exceptional circum- 
stances that such construction will prove economical. 
The same method has been applied to wood-block and 
stone-block pavements^ stone blocks being usually set 
in a bed of sand or gravel 4 to 8 inches deep. 

Art. 60. Choice of Foundation. 

It is always important that the foundation be suffi- 
cient. The yielding of the base of the pavement means 
its destruction. 

If a firm and durable foundation be employed, the 
surface may be renewed when necessary or changed 
from one material to another without disturbing the 
base, but if the base be weak the surface wiU be 
destroyed. 



FOUNDATIONS FOR PAVEMENTS. 221 

The saving of expense should be at the top rather 
than at the bottom of a pavement. 

The thickness required for the foundation of a pave- 
ment depends upon the nature of the soil upon which 
it is to rest, and upon the extent and weight of the 
travel to which it is to be subjected. 

When the road-bed is of a retentive material and 
likely to become wet and soft, the foundation should 
possess sufficient strength not to be broken through at 
points where the supporting power of the road-bed may 
be destroyed by water. It must also be able to resist 
the action of frost upon the soil below. In such cases 
8 or 9 inches of concrete may be necessary. Six inches 
of good concrete, however, constitute a foundation of 
considerable strength, and it is only under severe con- 
ditions, poor support and heavy traffic, that a greater 
depth is necessary. 

Under light traffic with good conditions, a less depth 
may be sometimes used; 4 inches of concrete is fre- 
quently employed to save expense, although 6 is the 
more common depth. A depth of 4 or 6 inches of well 
compacted gravel or broken stone is also usually suffi- 
cient vv^here the conditions are such as to admit of the 
use of a foundation of that character. 

It may be here observed that no definite prescrip- 
tion for any pavement, either as to choice of founda- 
tion or as to methods of construction, can fit all cases. 
What is most successful in one ^ase is quite inappli- 
cable in another. The blind following of particular 
rules b}^ those not conversant with the principles upon 
which they are based has been the cause' of many fail- 
ures. Judgment must always be used in weighing the 
local conditions of the problem in hand. 



CHAPTER VIII. 

BRICK PAVEMENTS. 
Art. 6i. Paving-brick. 

The requisites for a good paving-brick are that it 
shall be hard, tough, and impervious, as well as capable 
of enduring against the disintegrating influences of the 
weather. 

The bricks in most common use are made from fire- 
clay of an inferior quality, or from an indurated clay or 
shale of somewhat similar composition. 

These clays consist essentially of silicate of alumina, 
with usually small percentages of lime, magnesia, iron, 
potash, soda, and sometimes other elements. The 
range of composition for clays in common use is 
approximately as follows: 

Percent. 

Silica 60 to 75 

Alumina 10 to 25 

Iron oxide 3 to 8 

Lime to 4 

Magnesia to 3 

Potash 0.5 to 3 

Soda to 2 

In a few cases the quantity of lime is greater, vary- 
ing from 8 to 12 per cent. 

When the clay is very nearly pure silicate of alumina, 
it is capable of withstanding a high degree of heat 
without fusing, and is known as fire-clay. As the per- 

222 



BRICK PAVEMENTS. 223 

centages of other ingredients increase, it becomes more 
fusible. The lime, magnesia, potash, and soda act as 
fluxing agents, and the readiness with which the clay 
can be melted depends upon the relative quantities of 
refractory and fluxing materials that it may contain. 

Silica in excess tends to make the brick weak and 
brittle, while too great quantity of alumina causes the 
brick to crack and warp in the shrinking which occurs 
during burning. The proper adjustment of the rela- 
tions betv>^een these elements is necessar}^ to good 
results. 

The quantity of lime in the clay is an important 
matter, as the presence of lime in an uncombined state 
in the brick may be productive of disintegration when 
the brick is exposed to the weather. A large percent- 
age of lime in a clay is therefore to be regarded with 
suspicion, although not necessarily as cause for con- 
demnation, as its effect depends upon the state of com- 
bination of the various ingredients of the brick. Mag- 
nesia probably acts in much the same manner as lime. 
Potash and soda are considered desirable elements in 
quantities to properly flux the clay in burning. 

The fineness of a clay is also a matter of importance, 
both because a fine clay will fuse at a lower tempera- 
ture than a coarse one, and because fineness is neces- 
sary to the production of even and close grained brick, 
and therefore conduces to make them tough and 
impervious. 

To produce a good paving-brick, a clay is required 
which will vitrify at a high heat. A very refractory 
clay will make a porous brick, w^hile if it melts at too 
low a temperature it cannot be burned sufficienth^ to 
become hard and tough 

The methods of manufacturing paving-brick vary in 



224 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

different localities according to the character of the 
material to be worked. They are quite similar to 
those in use for common brick, only more thoroughly 
executed. 

The clay is commonly reduced to a fine powder, 
tempered with water and passed through a machine 
that molds the bricks, which are then dried and after- 
ward burned. Repressed bricks are those which are 
compressed in a mold after coming from the brick 
machine and before drying. 

The process of burning occupies usually from lO to 
15 days. 

The heat is at first slowly applied to expel the water, 
then raised to a high temperature for several days, 
after which the bricks are very slowly cooled. 

There is considerable difference of opinion among 
engineers and manufacturers as to the exact amount of 
burning necessary. It is usually stated that the brick 
should be burned to the point of vitrification, but not 
completely vitrified. The burning must be thoroughly 
done to produce a strong and impervious brick, but 
there is undoubtedly a point beyond which, for some 
brick, further burning causes brittleness. Verj^ gradual 
cooling is also necessary in order to toughen the brick. 
Smoothness and uniformity of texture in a paving 
brick is an important consideration as affecting its re- 
sistance both to crushing and to abrasion. The broken 
surface of the brick should present a uniform appear- 
ance both in texture and in color. 

All the bricks used in the same pavement should 
also be of the same degree of hardness and toughness 
in order that the pavement may wear evenly, and to 
this end careful inspection should always be given to 
the bricks proposed for use, and all of those which are 



BRICK PAVEMENTS. 225 

defective, soft from imperfect burning, brittle from 
everburning or quick cooling, cracked or distorted by- 
unequal shrinkage, should be rejected. An examina- 
tion of the color and size of the bricks may frequently 
be useful in determining for an}^ particular material 
whether individual bricks have received the proper 
degree of burning, after the engineer has become 
familiar with the make of brick under examination. 
The amount of shrinkage in burning is often a quite 
reliable index of the degree of burning to which the 
material has been subjected, and specifying within 
somewhat narrow limits the variation in size of bricks 
to be used together may often conduce to greater 
uniformity in the material employed. Some makes of 
brick vary quite appreciably in size for small differ- 
ences in extent of burning, and without materially 
affecting the value of the product, but it is desirable 
to sort them closely and use those of each size by 
themselves. 

The mistake is frequently made of placing too high 
value upon the element of hardness, which when car- 
ried to an extreme is sometimes attained at the ex- 
pense of toughness, the brick becoming brittle and 
easily shattered. The author (under his guaranty 
on a pavement) has, on one occasion, been obliged to 
replace a small number of hard bricks, which at the 
time of laying were supposed to be among the best 
of the lot, on account of their becoming shattered under 
traffic, while somewhat softer brick from the same kilns, 
the use of which was questioned bj^ the inspector, 
proved quite satisfactory in the same work. 

The sizes of paving-bricks vary considerably as 
made by different manufacturers, the most common 
sizes approximating to those of building brick, varying 



226 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

from about 2 J to 2|- inches in width, 8 to 8^ inches 
long, and 4 inches deep, about 56 to 63 bricks being 
required for a square yard of pavement. A few 
makers also produce a brick, of about the same size 
but only 3 J inches deep. A larger size, usually about 
3X9X4 inches, and known as a paving-block to 
distinguish it from the smaller brick, is quite largely 
employed. These usually require from 43 to 47 
blocks per square yard. 

Larger blocks have been tried, but have not come 
into general use, while some manufacturers make 
smaller sizes, requiring 70 to 75 bricks to the square 
yard of pavement. 

Opinions differ as to the best sizes for use in pave- 
ments, some engineers specifying the smaller bricks, 
others the larger blocks. A good pavement can 
be built of either if proper attention be given to 
selecting the material. The sizes preferred by the 
various manufacturers depend largely upon the 
character of the clay or shale with which they have to 
deal. With some materials the size is limited b\^ the 
distortion of large blocks in burning, and the smaller 
bricks are preferalle; with others, larger blocks may 
be made at less cost in proportion to area of pavement, 
and perhaps with better and smoother work resulting. 
There seems to be no necessity for an37^ increase in the 
usual depth of 4 inches as is sometimes proposed, and 
it may be possible that the adoption of the depth, t,^ 
inches, now frequently used may in many instances 
somewhat lessen the cost of the pavement without 
affecting its length of service. 

Much difference of opinion has been developed 
among engineers as to the advisability of rounding the 
corners of the brick, some requiring that the blocks be 



BRICK PAVEMENTS. 227 

repressed with comers rounded to a radius of i 01 | of 
an inch, while others specify square-edge brick, and 
in some instances that they shall not be repressed. 
On the one hand, it is maintained that in service the 
sharp comers will soon be laiocked off and worn 
down more roughlj^ and unevenl}^ than if originally 
rounded, while, on the other hand, it is claimed 
that if a rigid filler like Portland cement be used, the 
joint ma^^ be filled level with the surface of the brick, 
and be much less likely to chip out than if the joint 
be widened at the top so as to cause the filler to pre- 
sent a thin edge at the sides. Both contentions seem 
reasonable under certain conditions, and the method 
of construction and character of filler used will ordi- 
narily determine the proper form for the brick. 

The desirability of repressing the brick is also a 
much discussed question, it bemg argued b}^ some that 
the repressing of the material forms a more dense and 
compact block and increases its probable wear in use, 
and b3^ others that the pressure applied to the material 
after it comes from the brick-machine disturbs the 
structure and injures the fiber of the brick, often 
forming laminations which are elements of wealaiess. 
With some materials this last contention seems to 
have some basis in fact, but m other and probably 
most materials no such condition can be found on 
examination of the structure of the brick. The views 
of individual manufacturers upon the question seem to 
depend mainh^ upon the kind of material they have 
to work with, and it would be difficult from existing 
data to say that either method necessarily gives the 
best results. Possibh^ those materials which approach 
most nearh^ to actual vitrification and are subject to 
considerable shrinkage during the burning are but 



228 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

little affected in final density by the compression of 
the block before burning. It should be observed in 
this connection that whatever may be the value of 
repressing as to its effect upon the quality of the brick 
and wear of the pavement, it undoubtedly has the 
effect with some kinds of brick of giving a smoother 
and better surface to the pavement by producing a 
more regular and uniformly shaped brick. 

In order to give sufficient space between the bricks 
for the joint -filling, some manufacturers make re- 
pressed bricks with lugs on one side to hold them a 
given distance apart when laid in the pavement. The 
wisdom of this under ordinary circumstances seems 
doubtful, as small joint-space is usually desirable, and 
experience shows that bricks laid close, even if care- 
fully driven up, will usually give plenty of space for 
filling. Spacing-lugs are seldom required in specifica- 
tions, but engineers have sometimes required their use 
for work on steep grades with the idea of giving 
better foothold to horses than the thin joints would 
afford. There may be some advantage in this, and 
some pavements so constructed on grades of above 
eight per cent have given satisfactory results, the lugs 
usually projecting about half an inch from the face of 
the brick, but in the author's own experience he has 
been unable to notice any difference in the use of the 
pavement with or without the wide joints, all being 
laid with bevel-edge brick. 

Repressed bricks and blocks are frequently made 
with a groove or two, extending lengthwise of the 
brick on each side and sometimes across the ends, for 
the purpose of keying the blocks together when filled 
with the joint-filler. This may be an aid to rigid 
construction^ though not necessary to good work. 



BRICK PAVEMENTS. 229 

It is usually limited to the larger blocks, which fre- 
quently have thin lugs, as well as the grooves, and 
are known as groove- and lug-blocks. Thej^ are well 
calculated to give a very firm construction. 

Art. 62. Tests for Pavixg-brick. 

To determine the probable durability of brick 
designed for use in paving, mechanical tests may be 
applied which will show the relative rank of different 
samples in their most important characteristics. It is, 
however, a matter of considerable difficult}^ to set a 
standard to which the brick should be required to con- 
form, or to determine, from the behavior of the bricks 
under test, the relative value of various samples which 
it may be desired to compare. 

The tests ordinarily proposed or used for this purpose 
are those of crushing strength, transverse strength, 
abrasion and impact, absorption, and specific gravity. 
The relative importance of these tests and the weight 
which should be given to their results is a matter 
concerning which considerable difference of opinion has 
been developed amongst engineers, and practice va- 
ries considerably. The National Brick Manufacturers' 
Association have considered the matter, and in 1895 
appointed a committee which in 1897 reported a set 
of rules for making the tests, with resolutions expressing 
their views as to the relative importance and reliability 
of each. These rules, which were somewhat modified 
in 1900, are very commonh^ follow^ed and furnish a 
standard method of testing. 



230 A TEXT-BOOK ON ROADS AND PAVEMENTS. 
CRUSHING TEST. 

The recommendations of the commission for con- 
ducting this test are as follows: 

''I. The crushing test should be made on half- 
bricks, loaded edgewise, or as they are laid in the 
street. If the machine used is unable to crush a full 
half-brick, the area may be reduced by chipping off, 
keeping the form of the piece to be tested as nearly 
prismatic as possible. A machine of at least 100,000 
pounds capacity should be used, and the specimen 
should not be reduced below 4 square inches of area 
in cross-section at right angles to direction of load. 

"II. The upper and lower surfaces should prefer- 
ably be ground to true and parallel planes. If this is 
not done, they should be bedded in plaster of Paris 
while in the testing-machine, which should be allowed 
to harden ten minutes under the weight of the crush- 
ing planes only before the load is applied. 

"III. The load should be applied at a uniform rate 
of increase to the point of rupture. 

*' IV. Not less than an average obtained from 5 
tests, on 5 different bricks, shall constitute a stand- 
ard test/' 

The result of a compressive test of stone or brick 
depends very largely upon how it is made, and the 
results of tests are only properly comparable with 
others made in the same manner and with equal care. 
The use of plaster beds as suggested above, it is 
thought, conduces greatly to regularity of result in the 
work of different men, as it tends to reduce the effect 
of differences in the accuracy of dressing the surfaces 
of contact. The size of the test-piece is also impor- 
tant, the strength usually increasing as the size in- 



BRICK PAVEMENTS. 23 1 

creases. Small pieces, I J- or 2-inch cubes, are often 
employed because of the large force necessary to crush 
a whole or half brick, although where machinery exists 
capable of doing it the larger tests entail much less 
work in preparing specimens and also yield much 
more satisfactory results. Where small specimens are 
used it is to be observed that the unit strength will 
not be the same as for larger ones, and must be judged 
by a different standard. In the preparation of speci- 
mens it is better, when possible, to saw than to break 
them by chipping, in order not to injure the block by 
the shock of the blows. 

The commission in their discussion concluded that 
no connection has been show^n between high strength 
and the qualities necessary for a good paving material, 
and adopted the following resolution: 

''Whereas, From the experimental work done so 
far by this commission, or by others so far as is known 
to us, in the application of the cross-breaking and 
crushing tests to paving-bricks, it is not possible to 
show any close relationship between the qualities 
necessary for a good paving material and high struc- 
tural strength as indicated by either of these tests, 

"Resolved, That for this reason the commission rec- 
ommends that these tests shall be considered as purely 
optional in the examination of paving material, and 
not necessary as a proof of excellence/* 

It is to be observed that the actual crushing strength 
of a brick is not a matter of special importance in so 
far as any danger of the crushing of the material in the 
pavement is concerned, as no stress can there come 
upon it under ordinary circumstances which would 
endanger even a very weak specimen from direct 
crushing. It is thought, however, that to some extent 



232 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

the value of the brick is indicated by its resistance to 
crushing, coupled, of course, with a proper examina- 
tion of its other necessary attributes. A brick which 
possesses a high crushing strength is not necessarily 
a good paving-brick, as it may at the same time be 
brittle or of such composition as to easily disintegrate 
under the action of the weather; but one that yields 
to a low crushing strength is usually weak in wearing 
qualities and not fit for the purpose. For this reason 
this test is commonly included in specifications pre- 
scribing tests, although it is recognized that the rela- 
tive wearing qualities of various makes of brick can- 
not be graded by its results. A good paving-brick, 
in the form of a 2-inch cube, will usually show a re- 
sistance to crushing of not less than 10,000 pounds 
per square inch. Much higher values are sometimes 
used in specifications, but their advantage is at least 
doubtful. 

TRANSVERSE TEST. 

The transverse strength is tested by supporting the 
brick upon two knife-edges near its ends and bringing 
a load through a third knife-edge upon the middle of 
the brick. The test may be made upon any ordinary 
testing-machine by providing the necessary knife- 
edges, but, like the compression test, requires care in 
manipulation to get good results. It is specially im- 
portant that the brick have a perfectly even bearing 
upon the supports before the application of the load, 
in order that it may not be subjected to a twist under 
the load. The method adopted by the commission 
for this test is as follows : 

I. Support the brick on edge, or as laid in the 
pavement, on hardened steel knife-edges romided 



BRICK PAVEMENTS. 233 

longitudinally to a radius of 12 inches and trans- 
versely to a radius of one-eighth inch^ and bolted in 
position so as to secure a span of six inches. 

II. Apply the load to the middle of the top face 
through a hardened steel knife-edge, straight longi- 
tudinally and rounded transverselj^ to a radius of one- 
sixteenth inch. 

III. Apph^ the load at a uniform rate of increase 
till fracture ensues. 

IV. Compute the modulus of rupture by the for- 
mula 

^ 2 'hd?' 

in which / = modulus of rupture in pounds per square 
inch; 
w = total breaking load in pounds; 
/ = length of span in inches = 6; 
6 = breadth of brick in inches; 
d = depth of brick in inches. 

V. Samples for test must be free from all visible 
irregularities of surface or deformities of shape, and 
their upper and lower faces must be practically parallel. 

VI. Not less than 10 brick shall be broken and the 
average of all be taken for a standard test. 

The commission included this test with the crush- 
ing test in the recommendation that the test was to 
be considered optional and "not necessary as a proof 
of excellence. " This test is easier to conduct satis- 
factorilj^, and probably gives, in general, a more reli- 
able indication of the value of the material than the 
crushing test. It calls into plaj^ the tensile as well 
as compressive strength of the brick. The interior 
structure is showTi by the break, and an opportunity is 



234 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

given to judge of the uniformity and homogeneous 
character of the material. 

The fracture of a tough and homogeneous speci- 
men under a transverse load should be a clean break 
through the middle of the brick, and a close observa- 
tion of the breaks may frequently be of considerable 
assistance in forming an idea of these qualities, al- 
though they may not be directly represented by the 
load required to break the specimen. The shattering 
of the brick in breaking, or an irregular break extend- 
ing from the point of application of the load to one 
of the points of support, usually indicates brittleness 
of the material. 

The modulus of rupture of good paving-bricks 
commonly ranges between 2000 and 3000 pounds per 
square inch, sometimes reaching 3500 or even 4000 
pounds. It is usually somewhat greater for brick laid 
flat than for brick on edge. 

ABRASION TEST. 

In the convention of 1897 the Brick Manufacturers' 
Association adopted a method for this test consisting 
in rattling a given charge of bricks in a cylinder rotat- 
ing about its axis, which is horizontal, and depending 
for its result upon the impact and abrasion of the 
bricks upon each other. In 1900, however, after 
more fully considering the matter, the test was modi- 
fied, and a smaller charge of bricks, with the addition 
of a charge of cast-iron shot, was recommended as 
more nearly representing the conditions of practice 
and giving results more in accord with experience. 

The method finally recommended by the Associa- 
tion is as follows: 



BRICK PAVEMENTS. 235 

"I. Dimensions of the Machine, The standard 
machine shall be 2^ inches in diameter and 20 inches 
in length, measured inside the rattling-chamber. 

"Other machines may be used var\^ing in diameter 
between 26 and 30 inches, and in length from 18 to 
24 inches; but if this is done, a record of it must be 
attached to official report. Long rattlers maj^ be cut 
up into sections of suitable length by the insertion of 
an iron diaphragm at the proper point. 

"//. Construction of the Machine. The barrel shall 
be supported on trunnions at either end; in no 
case shall a shaft pass through the rattling-chamber. 
The cross-section of the barrel shall be a regular 
polygon having 14 sides. The heads and staves 
shall be composed of graj^ cast iron, not chilled or 
case-hardened. There shall be a space of one-fourth 
of an inch between the staves for the escape of dust 
and small pieces of waste. Other machines may be 
used having from 12 to 16 staves, with openings 
from one-eighth to three-eighths of an inch between 
staves; but if this is done, a record of it must be 
attached to the official report of the test. 

" IIL Composition of the Charge. All tests must 
be executed on charges containing but one make of 
brick or block at a time. The charge shall consist of 
9 paving-blocks or 12 paving-bricks, together with 
300 pounds of shot made of ordinary machinery cast 
iron. This shot shall be of two sizes, as described 
below, and the shot-charge shall be composed of one- 
fourth (75 pounds) of the larger size and three-fourths 
(225 pounds) of the smaller sizCo 

''IV, Size of the Shot. The larger size shall weigh 
about 7^ pounds and be about 2\ inches square and 
4J inches long, wdth slightly rounded edges. The 



236 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

smaller size shall be cubes of I J inches on a side, with 
rounded edges. The individual shot shall be replaced 
by new ones when they have lost one-tenth of their 
original weight. 

" V. Revolutions of the Charge. The number of 
revolutions of a standard test shall be 1800, and the 
speed of rotation shall not fall below 28 nor exceed 
30 per minute. The belt -power shall be sufficient to 
rotate the rattler at the same speed whether charged 
or empty, 

" VI. Condition of the Charge. The bricks com- 
posing the charge shall be dry and clean and as 
nearly as may be possible in the condition in which 
they were drawn from the kiln. 

'' VII. Calculation of the Result. The loss shall 
be calculated in the per cents of the weight of the 
dry brick composing the charge, and no result shall 
be considered as official unless it is the average of two 
distinct and complete tests made on separate charges 
of brick." 

The commission regard this as the most important 
test to be applied to paving-brick, and in fact it is 
the only one having their indorsement. It seems 
reasonable to suppose that this test gives more nearly 
than the others a determination of the value of the 
brick for use in a pavement. 

It is quite true that the action to which the brick 
is subjected in a test of this character is different 
from the wear to which it is subjected when firmly 
held in the pavement, but the qualities necessary to 
resist wear in the two cases are very similar. We 
may form an idea of whether a material is suitable for 
the proposed use from such experiments, although no 
definite idea of the amount of wear that it will en- 



BRICK PAVEMENTS. 237 

dure can be obtained from them. It should also be 
pointed out that the method of estimating the loss of 
the brick, from abrasion tests made in this manner, as 
percentages of the total weight of brick, can, in the 
comparison of different bricks, only give correct re- 
sults when the bricks compared are of the same size 
and shape. A brick with rounded edges evidently 
could not properh" be compared with one with sharp 
edges by this method, and some engineers have divided 
the test into two periods for the purpose of separat- 
ing the knocking off of the corners and preliminary 
rounding of the brick from the later abrasion upon 
the rounded surfaces which would be more nearly 
comparable for different specimens. If, in the test, 
the loss in the rattler during the first half-hour be 
separated from that during the second half-hour, 
the latter will be found to be much less affected by the 
form of the brick. 

In comparing bricks of different sizes it should be 
noted that a small brick presents more surface for 
abrasion than a large one in proportion to its volume, 
and the results of such comparisons would be con- 
siderabh^ modified in some instances if the results be 
stated in tenns of exposed surface instead of percent- 
age of volume. With square-edged brick during the 
early period of the test, when corners are being 
chipped off, the loss is probably more nearly propor- 
tional to length of edges than to surface or volume, 
which would be still more to the disadvantage of the 
small brick. Care is therefore necessar^^ in drawing 
conclusions from such tests concerning the relative 
values of different materials that all the conditions 
which may affect such conclusions be fully under- 
stood. 



238 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

New Abrasion Test, The abrasion test as above 
described, after being in use for a number of years, was 
found somewhat unsatisfactory, on account of the varia- 
tion in results obtained in different laboratories, and the 
National Paving Brick Manufacturers' Association, in 
191 1, recommended the adoption of a new machine, 
together with specifications for its use. These recommenda- 
tions are as follows: 

THE RATTLER. 

" The machine shall be of good mechanical construction, 
self-contained, and shall conform to the following details, 
of material and dimensions, and shall consist of barrel, 
frame and driving mechanism as herein described. 

THE BARREL. 

** The barrel of the machine shall be made up of the 
heads, headliners and staves. 

*' The heads shall be cast with trunnions in one piece. 
The trunnion bearings shall not be less than two and one- 
half (2 J) inches in diameter or less than six (6) inches in 
length. 

" The heads shall not be less than three-fourths (J) 
inch thick nor more than seven-eighths (|) inch. In out- 
line they shall be a regular fourteen-sided (14) polygon 
inscribed in a circle twenty-eight and three-eighths (23I) 
inches in diameter. The heads shall be provided with 
flanges not less than three-fourths (J) inch thick and 
extending outward two and one-half (2 J) inches from 
he inside face of head to afford a means of fastening the 
staves. The flanges shall be slotted on the outer edge, 
so as to provide for two (2) three-fourths (J) inch bolts 
at each end of each stave, said slots to be thirteen-sixteenths 



BRICK PAVE]MENTS. 239 

(yI) inch wide and two and three-fourths (2f) inches 
center to center. Under each section of the flanges there 
shall be a brace three-eighths (|) inch thick and extending 
down the outside of the head not less than two (2) inches. 
Each slot shall be provided with recess for bolt head, which 
shall act to prevent the turning of the same. There shall 
be for each head a cast-iron headliner one (i) inch in 
thickness and conforming to the outline of the head, 
but inscribed in a circle twenty-eight and one-eighth (2 8 J) 
inches in diameter. This liner or wear plate shall be 
fastened to the head by seven (7) five-eighths (t) inch 
cap screws, through the head from the outside. These 
wear plates, whenever they become worn down one-half 
(J) inch below their initial surface level, at any point of 
their surface, must be replaced with new. The metal 
of which these wear plates are to be composed shall be 
what is known as hard machinery iron, and must con- 
tain not less than one (i) per cent of combined carbon. 
The faces of the polygon must be smooth and give uni- 
form bearing for the staves. To secure the desired 
uniform bearing the faces of the head may be ground 
or machined. 

THE STAVES. 

" The staves shall be made of six (6) inch medium 
steel structural channels twenty-seven and one-fourth 
(2 7 J) inches long and weighing fifteen and five- tenths 
(15.5) pounds per lineal foot. 

'' The channels shall be drilled with holes thirteen- 
sixteenths (f|) inch in diameter, two (2) in each end, 
for bolts to fasten same to head, the center line of the 
holes being one (i) inch from either end and one and 
three-eighths (if) inches either way from the longitudinal 
center line. 



240 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

" The spaces between the staves will be determined 
by the accuracy of the heads, but must not exceed five- 
sixteenths (j^) inch. The interior or fiat side of each 
channel must be protected by a lining or wear plate 
three-eighths (|) inch thick by five and one-half (5 J) 
inches wide by nineteen and three-fourths (19!) inches 
long. The wear plate shall consist of medium steel 
plate, and shall be riveted to the channel by three (3) 
one-half (J) inch rivets, one of which shall be on the 
center line both ways and the other two on the longitudinal 
center line and spaced seven (7) inches from the center 
each way. The rivet holes shall be countersunk on 
the face of the wear plate and the rivets shall be driven 
hot and chipped off flush with the surface of the wear 
plate. These wear plates shall be inspected from time 
to time, and if found loose shall be at once reriveted, 
but no wear plate shall be replaced by a new one except 
as the whole set is changed. No set of wear plates shall 
be used for more than one hundred and fifty (150) tests 
under any circumstances. The record must show the 
date when each set of wear plates goes into service and the 
number of tests made upon each set. 

" The staves when bolted to the head shall form a 
barrel twenty (20) inches long, inside measurement, 
between wear plates. The wear plates of the staves 
must be so placed as to drop between the wear plates of 
the heads. These staves shall be bolted tightly to the 
heads by four (4) three-fourths (f) inch bolts, and each 
bolt shall be provided with lock nuts, and shall be inspected 
at not less frequent intervals than every fifth (5th) test 
and all nuts kept tight. A record shall be made after 
each such inspection, showing in what condition the bolts 
were found. 



BRICK PAVEMENTS. 241 

THE FRAME AND DRIVING MECHANISM. 

'* The barrel shall be mounted on a cast-iron frame of 
sufficient strength and rigidity to support same without 
undue vibration. It should rest on a rigid foundation 
and be fastened to same by bolts at not less than four 
(4) points. 

*' It should be driven by gearing whose ratio of driver 
to driven should not be less than one (i) to four (4). 
The countershaft upon which the driving pinion is 
mounted should not be less than one and fifteen-sixteenths 
(lyf) inches in diameter with bearings not less than six 
(6) inches in length and belt driven, and the pulley should 
not be less than eighten (18) inches in diameter and six 
and one-half (6J) inches in face. A belt of six (6) inch 
double-strength leather, properly adjusted, so as to avoid 
unnecessary slipping, should be used. 

" The National Paving Brick Manufacturers' Associa- 
tion will furnish without charge to all proper applicants 
the complete drawings of a machine which will meet the 
above specifications and requirements. 

THE ABRASIVE CHARGE. 

" (a) The abrasive charge shall consist of two sizes 
of cast-iron spheres. The larger size shall be three and 
seventy-five hundredths (3.75) inches in diameter when 
new and shall weigh when new approximately seven and 
five-tenths (7.5) pounds (3.40 kilos) each. Ten shall be 
used. 

" These shall be weighed separately after each ten 
(10) tests, and if the weight of any large shot falls to 
seven (7) pounds (3.175 kilos) it shall be discarded and 
a new one substituted; provided, however, that all of 



242 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

the large shot shall not be discarded and substituted by 
new ones at any single time, and that so far as possible 
the large shots shall compose a graduated series in various 
stages of wear. 

" The smaller size spheres shall be when new one 
and eight hundred seventy-five thousandths (1.875) inches 
in diameter and shall weigh not to exceed ninety-five 
hundredths (.95) pound (0.430 kilo) each. Of these 
spheres so many shall be used as will bring the collective 
weight of the large and small spheres most nearly to 
three hundred (300) pounds, provided that no small 
sphere shall be retained in use after it has been worn 
down so that it will pass a circular hole one and seventy- 
five hundredths (1.75) inches in diameter, drilled in a 
cast-iron plate one-fourth (-J) inch in thickness or weigh 
less than seven ty-five-hundredths (.75) pound (or .34 
kilo) . Further the small spheres shall be tested by passing 
them over such an iron plate drilled with such holes, 
or shall be weighed after every ten (10) tests, and any 
which pass through or fall below specified weight, shall 
be replaced by new spheres, and provided, further, that 
all of the small spheres shall not be rejected and replaced 
by new ones at any one time, and that so far as possible 
the small spheres shall compose a graduated series in 
various stages of wear. At any time that any sphere is 
found to be broken or defective it shall at once be replaced. 
*' (b) The iron composing these spheres shall have a 
chemical composition within the following limits : 

Combined carbon- — Not less than 2.50 per cent. 

Graphitic carbon — Not more than 0.20 per cent. 

Silicon' — Not more than i per cent. 

Manganese — Not more than 0.50 per cent. 

Phosphorus^ — Not more than 0.25 per cent. 

Sulphur- — Not more than 0.08 per cent. 



BRICK PAVEMENTS. 243 

" For each new batch of spheres used the chemical 
analysis must be furnished by the maker, or be obtained 
by the user, before introduction into the charge, and 
unless the analysis meets the above specifications, the 
batch of spheres shall be rejected. 

THE BRICK CHARGE 

" The number of brick per charge shall be ten (10) 
for all bricks of the so-called ' block size ' whose dimen- 
sions fall between from eight (8) to nine (9) inches in 
length, three (3) and three and three-fourths (3!) inches 
in breadth and three and three-fourths (3J) and four and 
one-fourth (4-J-) inches in thickness. No block should be 
selected for test that would be rejected by any other re- 
quirements of the specifications. 

The brick shall be clean and dried for at least three 
(3) hours in a temperature of one hundred (100) degrees 
F. before testing. 

THE TEST 

'' The rattler shall be rotated at a uniform rate of not 
less than twenty-nine and one-half (29J) nor more than 
thirty and one-half (30J) revolutions per minute, and 
eighteen hundred (1800) revolutions shall constitute the 
standard test. 

''A counting machine shall be attached to the rattler 
for counting the revolutions. 

"A margin of not to exceed ten (10) revolutions will 
be allowed for stopping. 

STOPPING AND STARTING 

"Only one (i) start and stop per test is regular and 
acceptable. 



244 A TEXT-BOOK ON ROADS AND PAVEMENTS. 
THE RESULTS 

" The loss shall be calculated in percentage of the 
original weight of the dried brick composing the charge. 
In weighing the rattled brick any piece weighing less 
than one (i) pound shall be rejected. 

RECORDS 

" (a) The operator shall keep an official book, in which 
the alternate pages are perforated for removal. The 
record shall be kept in duplicate, by use of a carbon 
paper between the first and second sheets, and when 
all entries are made and calculations are completed the 
original record shall be removed and the carbon duplicate 
preserved in the book. All calculations must be made 
in the space left for that purpose in the record blank, 
and the actual figures must appear. The record must 
bear its serial number, and be filled out completely for 
each test, and all data as to dates of inspection and weighing 
of shot and replacement of worn-out parts must be care- 
fally entered, so that the records remaining in the book 
constitute a continuous one. In event of further copies 
of a record being needed, they may be furnished on separate 
sheets, but in no case shall the original carbon copy be 
removed from the record book. 

" (b) The blank form upon which the record of all 
official brick tests is to be kept and reported is as follows: 



BRICK PAVEMENTS. 



245 



REPORT OF STANDARD RATTLER TEST OF PAVING 

BRICKS. 

Identification Data. 

Name of the firm furnishing sample Serial No. ( ) 

Name of the firm manufacturing sample 

Street or job which sample represents 

Brands or marks on the brick 

Quantity furnished Drying treatment 

Date received Date tested 

Length Breadth Thickness 

Standardization Data. 
Number of charges tested since last inspection 



Weight of Charge 
(After Standardization). 


Condition of Locknuts 
on Staves. 


Condition of Scales. 


10 Large spheres 

Small spheres 






Total 











Number of charges tested since stave linings were renewed .... 
Repairs (Note any repairs affecting the condition of the barrel) 

Running Data. 





Time Readings. 


Revolution 
Counter 
Readings. 


Running Notes. 




Hours 


Min. 


Sec. 




Beginning of test 
Final reading . . . 












Weights and Calculations. 



Initial weight of 10 bricks. 

Final weight of same 

Loss of weight 



Percentage loss 

(Note. — The calculation 

must appear) 



Number of broken bricks and remarks on same 

I certify that the foregoing test was made under the specifications 

of and is a true record. 

(Signature of Tester) 



Date Location of Laboratory 



246 A TEXT-BOOK ON ROADS AND PAVEMENTS. 



ABSORPTION TEST. 

This test is made by weighing the specimen dry, 
then saturating it with water, weighing again, and 
stating the absorption as a percentage of the dry 
weight. The Commission of the Brick Manufacturers' 
Association oppose the use of this test, but recommend 
the following procedure for the test when used: 

I. The number of bricks for a standard test shall 
be 5. 

II. The test must be conducted on rattled bricks. 
If none such are available, the whole bricks must be 
broken in halves before treatment. 

III. Dry the bricks for 48 hours at a temperature 
ranging from 230 degrees to 250 degrees F. before 
weighing for the initial dry weight, 

IV. Soak for 48 hours, completely immersing the 
brick. 

V. After soaking and before reweighing wipe the 
brick until free from surplus water and practically 
dry on the surface. 

VI. Reweigh the samples at once on scales which 
are sensitive to I gram. 

VII. The increase in weight due to absorption is to 
be calculated in percentage of the dry weight of the 
original bricks. 

The commission also adopted the following resolu- 
tion : 

" Resolved, That, in the opinion of the commission, 
any paving-brick which will satisfy the requirements 
of reasonable mechanical tests will not absorb sufficient 
water to prove injurious to it in service. We therefore 
recommend that the absorption test be abandoned as 
unnecessary, if not actually misleading.'' 



BRICK PAVEMENTS. 247 

The purpose of this test, when made, is to insure 
the proper burning of the brick to a compact and non- 
absorbent structure. It is probable, as claimed by the 
commission, that these qualities will alwaj'S be shown 
by the other tests, and that this one is not of very 
great importance, but in many instances it may give 
useful information. A good paving-brick will not 
usually absorb more than 4 per cent or 5 per cent of 
water, but the amount of absorption depends largely 
upon the nature of the material from which the brick 
is made. Many of the shale bricks absorb less than 
I per cent of water if properly burned, while some of 
the so-called fire-clay bricks when of equall}^ good 
quality will absorb 3 per cent or 4 per cent. A 
specification which would insure the proper burning 
of the one class would exclude the best of the other 
class. A limit to the amount of absorption allowable 
is, however, commonly set in specifications. 

The requirement of drying 48 hours is probably, in 
most instances, sufficient and reduces the moisture in 
the brick so that the further loss from continued 
drying w^ould be very slight, but the saturation of the 
brick will not usually be accomplished by immersing 
for 48 hours. Some bricks will in that time have 
taken up but a small part of the water they would 
finally absorb, and much, longer time w^ould be neces- 
sary to give a complete indication in this particular. 
Some experiments by I\Ir. Harrington of St. Louis, 
the results of which were presented to the Brick 
Manufacturers' Association, showed a considerable 
change in the quantity of water absorbed by some 
bricks through a period of 24 weeks, and a consider- 
able variation in the rate of absorption by different 
bricks. 



248 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

The application of the test as proposed may serve 
to show whether the absorption is within the proper 
limits for a paving-brick, and when properly applied 
to particular makes of brick may indicate the degree 
of burning, although it is not of much value in the 
comparison of the qualities of different bricks where 
each shows results within reasonable limits. 

SPECIFIC-GRAVITY TEST. 

A test for specific gravity is sometimes included 
in specifications with a view to insuring the burning of 
the brick to the proper density. 

In consequence, however, of the variation of bricks 
made from different materials it does not seem feasible 
to adopt any requirement foi general use which would 
be of much value, and as the same qualities are de- 
termined b7 other tests it seems unnecessary. The 
Commission of the Brick Manufacturers' Association 
recommend the abandonment of this test. 

COMPARISON OF TESTS. 

Specifications for brick pavements commonly require 
that the brick reach certain limiting values on some of 
the tests which have been described. The crushing test 
and the specific gravity test, while useful sometimes in a" 
study of the character of materials, are of no value in 
specifications and are seldom used. Absorption is some- 
times specified, a limit of 2 to 4 per cent being given for 
the shale brick, but is a doubtful value as a measure of 
quality. The transverse test is frequently used, requiring 
about 1800 to 2000 pounds per square inch as modulus 
of rupture by the standard test. 



BRICK PAVEMENTS 249 

The main dependence, in specifications, is usually 
placed upon the rattler test, which is without doubt the 
most important, as showing more nearly than the others 
the qualities necessary in a brick to give good wear 
under traffic in a pavement, and many specifications now 
require no odier test for acceptance of the brick. Speci- 
fications vary in the percentage of loss allowed, in testing 
by the old method, from about 18 to 27 per cent, according 
to brick available in the locality, the intensity of traffic, 
and the experience of the engineer with the test. In 
adopting the new test the American Society of Municipal 
Improvements recommend that the limit of loss of weight 
be fixed at 22 per cent for heavy, 26 per cent for medium, 
and 28 per cent for light traffic streets. The Association 
for Standardizing Paving Specifications recommends that 
the brick shall not lose more than 22 per cent of their 
weight, but provides that " where medium or light traffic 
or other conditions which in the opinion of the engineer 
do not require a brick sufficient to stand an abrasion 
loss of 22 per cent, brick of a quahty sufficient to stand 
a loss of 25 per cent or even 28 per cent may be used." 

Art. 63. Construction of Brick Pavements. 

The work to be perfonned in laying a brick pave- 
ment, after grading and rolling the road bed (see 
Art. 53), consists in placing the foundation; forming a 
cushion coat of sand over the foundation; lajdng the 
bricks upon the sand cushion; rolling, or ramming, the 
bricks to a uniform surface and bearing; culling all 
broken and imperfect bricks ; filling the joints between 
the bricks; cleaning the pavement and opening it to 
traffic. 

Foundation. A brick pavement should have a firm 



250 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

foundation. As the surface is made up of small inde- 
pendent blocks, each brick must be adequately sup- 
ported from below, or the loads coming upon it may 
force it downward and cause unevenness. The wear of 
the pavement depends very largely upon the mainten- 
ance of a smooth even surface, as any unevenness will 
cause the bricks to chip on the edges, and also produce 
impact from the loads passing over the pavement „ 

The best foundation for a brick pavement is doubt- 
less one of concrete, laid after the manner given in 
Art. 57. For light or moderately heavy traffic, such as 
that of the ordinary small city where the road-bed is 
of firm soil and properly drained, the concrete is usu- 
ally placed 4 to 6 inches thick. If the traffic be very 
heavy or where from any cause the road-bed is not firm, 
it may be advisable to still farther increase the depth. 

The double-layer pavement (see Fig. 24) consists of 
a foundation made by placing a layer of sand or gravel 
3 to 5 inches thick upon the road-bed, rolling it 
thoroughly and laying a course of bricks upon it. The 
bricks are laid fiat with their greatest dimension length- 
wise of the street, as explained in Art. 59. This founda- 
tion has been extensively used under brick pavements, 
and has often given satisfactory results. It is now largely 
giving place to concrete in the better class of work, and 
in many cases under light traffic its economy is question- 
able, as the layer of gravel would often answer equally 
well without the lower layer of bricks. A modification of 
this base has been used in a few instances, in which the 
joints between the bricks are filled with Portland cement 
mortar. When such a base can be placed cheaper than 
concrete, it may give good service. The National Paving 
Brick Manufacturers' Association propose the following 
method of construction : 



BRICK PAVEMENTS. 251 

'' Upon the sub-grade as heretofore prepared shall be 
spread a base of sand two (2) inches in thickness and 
which shall be brought to a perfect grade, conforming 
to that of the finished street. 

" There shall be laid flatwise at right angles with the 
street, upon this grade thus prepared, a layer of No. 2 
Paving Block not less than three (3) inches in thickness, 
the interstices of which shall be filled with a filler com- 
posed of two parts of clean sand and one part of Portland 
cement. This filler shall be prepared and applied as 
provided for in Section Ten of this direction and specifi- 
cation. The foundation thus made should remain undis- 
turbed at least thirty-six (36) hours before the sand 
cushion herein provided for may be spread, and at least 
ten (10) days must elapse before rolhng and compacting 
of brick surface is allowed, and in no event must teams 
be permitted or hauling be allowed upon this surface 
during this period." 

Sand Cushion. The sand cushion consists usually 
of a layer of sand varying, in the practice of dift'erent 
engineers, from i to 2J inches in thickness over the 
surface of the foundation. The most common prac- 
tice is to make the sand cushion 2 inches thick. It 
should be deep enough to admit of the brick being 
driven to a smooth surface, and to take up any in- 
equalities in the surface of the foundation and differ- 
ences in thickness of brick. When a very thin cushion 
laj^er is emploj'cd it is necessary to secure much 
greater accuracy in forming the surface of the founda- 
tion, and it is much more difficult to uniforml}^ bed 
the bricks than when the usual thickness is used. It 
has also been claimed that the thicker sand-bed has 
a marked tendencj" to diminish the rumbling of the 
pavement. This, however, is perhaps rather doubtful- 



252 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

In forming the sand-bed the sand is spread over the 
foundation a httle deeper than the bed is to be left, and 
is then drawn off to a smooth surface by the use 
of a form, cut to the desired shape of the surface, 
which extends across the street, and shdes on the 
curbs or on stringers laid lengthwise of the street as 
may be convenient. The making of a good firm bed 
requires that considerable sand shall be pulled off 
from all parts of the bed, and the sand should always 
be two or three inches deep against the front of the 
form when drawing it to cut the bed. In order to 
accomplish this, without failing to cut a perfect sur- 
face through the form leaving the guides and riding 
up on the sand, considerable weight is required on the 
form when drawing it. In some cases the sand cushion 
is rolled with a light roller after being formed with a 
template. The specifications of the Association for 
Standardizing Paving Specifications require that this be 
done. 

The sand for a cushion should be clean and free 
from pebbles, which prevent the formation of a smooth 
bed, and possibly also cause the breaking of the bricks 
in ramming the surface. 

When the sand cushion is I J or 2 inches deep, an 
allowance of about half an inch is necessary for settle- 
ment in driving the bricks to surface. 

Laying the Brick. The bricks in a street pavement 
are usually laid on edge in courses across the street, 
each alternate course being begun with a half-brick to 
break joints in the courses. This is illustrated in 
Fig. 23, which represents a pavement as constructed for 
heavy traffic on concrete foundation. 

In many cases the gutter-bricks are turned with the 
greatest dimension lengthwise of the street, with the 



BRICK PAVEMENTS. 



253 



object of facilitating the flow of surface-water in 
the gutter. The advantage of this is doubtful, as it 




Fig. 23. 

has the effect of breaking the bond of the pavement 
between the gutter-bricks and roadway« This is shown 
in Fig. 24, which represents the construction of a 




.^^^m 



^ X 



i ^. X J-....L.J........LJ...-J...- . L J 









mmm^^m^^ 




Fig. 24. 

double-layer pavement with brick and gravel base, as 
has been commonly used under light or moderate 
traffic. 

In laying the bricks the men stand on the pavement 
alread3^ laid and, beginning at a curb, lay three or four 
courses across the street at once, the . bricks being 
wheeled and piled on the edge of the finished w^ork 



254 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

by laborers working continually in advance of the men 
laying. Wheeling over the newly laid bricks should 
be done on planks, to prevent driving the bricks out 
of surface. The courses may be kept straight and 
close together by driving back each block of eight or 
ten courses, a straight piece of plank four or five feet 
long being held by a handle on top against the side of 
the last course of bricks, and tapped lightly with a sledge. 

Surfacing the Pavement. After the surface layer of 
bricks is in position it should be swept clean and rammed 
or rolled to a smooth and uniform surface. A five- 
or six- ton roller may be employed, passing three or 
four times over the surface, or a wooden rammer loaded 
with lead to a weight of 80 or 100 pounds may be used 
by striking upon a plank laid lengthwise of the street. 
The plank should be 10 or 12 feet long by about a foot 
wide and 3 inches thick, and used only so long as it 
retains its form and solidity. 

The National Paving Brick Manufacturers' Association 
recommends the following procedure in compacting and 
smoothing the surface of the pavement: 

" After the brick in the pavement are inspected and 
the surface is swept clean of spalls, they must be well 
rolled, with a steam roller weighing not less than three 
nor more than five tons, in the following manner: The 
brick next the curb should be tamped with a hand wood 
tamper to the proper gutter grade. The rolling will 
then commence near the curb at a very slow pace and 
continue back and forth until the center of the pavement 
is reached, then pass to the opposite curb and repeat 
in the same manner to the center of the street. After 
this first passage of the roller the pace may be quickened 
and the rolling continued until each brick is firmly 
imbedded in the sand cushion. The roller shall then he 



BRICK PAVEMENTS. 255 

started at the end of the block and the pavement rolled 
transversely at an angle of 45 degrees to curb, repeat the 
rolling in like manner in the opposite direction. Before 
this transverse rolling takes place all broken or injured 
brick must be taken up and replaced with perfect ones." 

When the pavement has been brought to surface a 
careful inspection should be made and all defective 
or broken brick removed and replaced, a pair of brick- 
tongs being used for the purpose, and all low bricks 
or low spots being raised and brought to surface. A 
straight-edge is desirable in determining surface, as 
the appearance is often deceptive to the eye, and a 
slight variation in color of brick is frequently mistaken 
for an irregularity of surface. Sometimes the pavement 
is sprinkled and soft bricks picked out by observing 
whether they hold moisture; this method should be 
used w^ith caution and with full knowledge of the 
material, as sometimes a comparatively slight absorp- 
tion will show quite markedly. 



Art. 64. Filling the Joints. 

There is much difference of opinion amongst munici- 
pal engineers concerning the best material to use in 
filling the joints in a brick pavement. The materials 
commonly emploj^ed are sand, Portland-cement grout, 
and asphaltic or coal-tar paving-cement. Certain patent 
fillers of more or less the same character are also some- 
times employed. 

Sand Filler. In using sand as a filler, a thin layer 
of sand is spread over the pavement and raked or 
swept into the joints until they are thoroughly filled. 
In some instances the sand is artificially dried before 
putting it upon the bricks, but ordinarily, in mod- 



256 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

erately dry weather, the sand may be spread in a thin 
layer on the bricks and allowed to dry before sweep- 
ing in. After the joints are well filled a light layer 
of sand is placed on top of the pavement and it is 
opened to traffic. The jarring of the traffic will 
cause the sand to settle more or less in the joints for 
a considerable time, and the sand cover should be 
retained for several weeks. The sand used for filling 
should be fine and sharp, free from loam and dirt. It 
must not pack or cake on top of the brick under traffic. 
In many instances sand has been used as a filler with 
satisfactory results and given good service even under 
moderately heavy traffic. It makes a practically 
impervious joint and holds the bricks quite firmly in 
place„ It seems desirable in the use of sand filler to 
employ round-edge bricks, as the edges are not held so 
firmly as with a rigid filler, and if sharp are more likely 
to be chipped off, while the round edges aid in thoroughly 
filling the joints with the sand. 

Portland-cement Filler. For filling joints with Port- 
land cement a grout composed of equal parts of cement 
and sand is commonly employed. This grout is mixed 
in a tight box to a condition such that it will readily 
flow into the joints, and is swept in with brooms until 
the joints are thoroughly filled. The easiest method 
of securing the complete filling of the joint is probably 
that of applying the grout in two parts, mixing the first 
part very wet and filling the joints nearl}^ full. As the 
grout begins to stiffen, the draining out of the water 
causes it to settle somewhat, but thoroughly fills and 
seals the lower part of the joint. The second part may 
then be mixed a little stiffer and the upper part of 
the joint be readily filled flush with the top of the 
bricks. ,. _ _ ^ 



'BRICK PAVEMENTS. 257 

In handling the grout it is necessary that it be mixed 
quickly and applied at once without giving it time to 
settle, in order that it may retain its consistency and 
no separation of its materials take place. A pavement 
so filled becomes practically a monolithic mass, as the 
bricks are firmly held together and the joint is filled 
flush with the edges of the bricks with a material which 
soon becomes about as hard as the bricks themselves. 
The National Paving Brick Manufacturers' Association 
advocates the exclusive use of this filler, and recommends 
the following method of applying it : 

" The filler shall be composed of one part each of clean, 
sharp sand and Portland cement. The sand should be 
dry. The mixture, not exceeding one-third bushel of 
the sand, together with a like amount of cement, shall 
be placed in the box and mixed dry, until the mass 
assumes an even and unbroken shade. Then water 
shall be added, forming a liquid mixture of the consistency 
of thin cream. 

" The side and edges of the brick should be thoroughly 
wet before the filler is applied by being gently sprinkled. 

" From the time the water is applied until the last 
drop is removed and floated into the joints of the brick 
pavement, the mixture must be kept in constant motion. 

'' The mixture shall be removed from the box to the 
street surface with a scoop shovel, all the while being 
stirred in the box as the same is being thus emptied. 
The box for this purpose shall be 4 feet 8 inches long, 
30 inches wide and 14 inches deep, resting on legs of 
dift'erent lengths, so that the mktures will readily flow 
to the lower corner of the box, the bottom of which should 
be 6 inches above the pavement. This mixture, from 
the moment it touches the brick shah be thoroughly 
swept into the joints. 



258 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

" Two such boxes shall be provided in case the street 
is twenty feet or less in width; exceeding twenty feet in 
width, three boxes should be used. (See specifications 
for making same.) 

" The work of filling should thus be carried forward 
in line until an advance of fifteen to twenty yards has 
been made, when the same force and appliances shall 
be turned back and cover the same space in like manner, 
except to make the proportions two-thirds Portland cement 
and one-third sand. 

" To avoid the possibility of thickening at any point 
there should be a man with a sprinkling can, the head 
perforated with small holes, sprinkling gently the surface 
ahead of the sweepers. 

" Within one-half to three-quarters of an hour after 
this last coat is applied and the grout between the joints 
has fully subsided and the initial set is taking place, 
the whole surface must be shghtly sprinkled and all 
surplus mixture left on the tops of the brick swept into 
the joints, bringing them up flush and full. 

" After the joints are thus filled flush with the top of 
the brick and sufiicient time for hardening has elapsed, 
so that the coating of sand will not absorb any moisture 
from the cement mixture, one-half inch of sand shall 
be spread over the whole surface, and in case the work 
is subjected to a hot summer sun, an occasional sprinkling, 
sufficient to dampen the sand, should be followed for 
two or three days." 

When using Portland cement filler, an expansion joint 
must be placed next the curb on each side of the street, 
to prevent the brick surface being lifted from the sand 
cushion by expansion. This joint is made by placing 
strips of wood along the curb, against which the brick 
may be set, and after the Portland cement filler has been 



BRICK PAVEMENTS. 259 

put in, the boards are removed and the expansion joints 
filled with asphaltic or coal tar cement. 

Sometimes expansion joints are also used at intervals 
across the street to provide for longitudinal expansion. 
To be efficient, these joints should not be less than | 
inch, when spaced 50 feet apart. They will usually be 
nearly closed in a few months, the paving pitch being 
forced out by the expansion. Such joints are not usually 
necessary on pavements of uniform gradients, but where 
changes occur in the rate of grade, and at intersections 
expansion must be carefully provided for. After com- 
pletion, the pavement should stand at least a week or 
ten days before it is opened to traffic, to allow sufficient 
time for the cement to harden. During this time it 
should not be exposed to hot sun or permitted to dry 
out too much; commonly a light coating of sand is spread 
over it, and sometimes it is dampened by frequent 
sprinklings. 

Bituminous Filler. The bituminous cements employed 
as fillers for brick pavements may be prepared from coal 
tar or petroleum residuums, or from natural asphalt, 
and are similar in character to the materials used for 
bituminous macadam or in the preparation of asphalt 
surface mixtures. 

On account of the variation in the materials used, 
the results obtained with bituminous fillers have differed 
widely in dift'erent places. In many cases very satis- 
factory results have been obtained with these fillers; 
in others, the susceptibility to temperature changes has 
made them failures, the material melting and running 
out of the joints in hot weather, or becoming brittle and 
chipping out in cold weather. 

Bituminous fillers, when of good materials and properly 
apphed, give very satisfactory results. They have the 



26o A TEXT-BOOK ON ROADS AND PAVEMENTS. 

advantage over sand of being quite impervious to water, 
while they are not so rigid as Portland cement filler 
and are less noisy. These materials are under the dis- 
advantage that well-defined specifications, which will 
insure the quality of the material, have not been worked 
out. Most of the tar fillers which have been on the 
market are too susceptible to changes of temperature. 
Some of the asphaltic fillers have given good results. 
For asphaltic fillers, the Association for Standardizing 
Paving Specifications recommends the following require- 
ments : 

" The interstices in the brick shall be completely filled 
with an asphalt filler heated to a temperature of not less 
than 350° F. nor more than 450° F. This asphalt filler 
shall not contain pitch nor any part of coal tar. It 
shall contain at least 92 per cent of bitumen soluble in 
carbon disulphide. It shall remain pliable at all tem- 
peratures to which it may be subjected as a street 
paving filler; it shall be absolutely proof against water 
and street Hquids; it shall firmly adhere to the brick 
and be pliable rather than rigid. Care shall be exercised 
to completely fill all openings around street structures 
and the street shall not be used for traffic until the 
filler is completely set. A top dressing of sand shall be 
spread immediately after the filler is appKed and while 
it is still soft." 

" The penetration shall conform to the following: 

"No. 2 needle 5 sec. 100 grams at 77 degrees F., 25 to 60. 
"No. 2 needle i min. 200 grams at 32 degrees F., not below 25. 
"No. 2 needle 5 sec. 50 grams at 115 degrees F., not below no," 

When bituminous filler is employed it is melted in 
kettles on the street and poured hot into the joints. 
The paving-cement is appHed at a temperature of 300° 



BRICK PAVEMENTS. 261 

to 400° F., and should be applied only when the bricks 
are dry. After the joints are filled a light layer of sand 
is spread over the surface, and serves under the traffic 
to clean the surface from any surplus bitumen which 
may be smeared over it. 

Art. 65. Maintenance of Brick Pavements. 

The maintenance necessary for a brick pavement 
consists in keeping it clean and carefully watching it, 
especially during the first year or two years, to see that 
no breaks occur due to the use of defective bricks in 
the surface or to insufficient support from the founda- 
tion at any point. When any unevenness from either 
of these causes appears, it should be at once rectified 
before the pavement becomes irregularly worn in con- 
sequence. 

While, as already stated, the utmost care should 
always be taken to use only material of a uniform 
quality in the surface of the pavement, still under the 
closest inspection some inferior material may be used, 
which will only be shown when wear comes on the 
pavement, and unless then removed at once it will 
cause the evenness of the surface to be impaired about 
it. Irregular support from the foundation will be 
less likely to occur in good construction, but its effect 
will be similar to defective material, the sinking of in- 
dividual bricks producing uneven wear. Weak spots 
in the foundation may sometimes be caused, where con- 
crete foundation is not emploj^ed, by surface-water 
which is permitted to pass through the joints, saturat- 
ing the sand or gravel beneath and causing it to move 
under concentrated loads. For this reason the joints 
ghould be observed during the early w^ear of the pave- 



262 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

merit in order to remedy any case where they may not 
have been properly filled. 

Where a brick pavement has been constructed of 
good material and kept in good surface during the 
early period of use^ it may then reasonably be expected 
to wear out without any considerable expense for small 
repairs. The length of time the pavement may be 
expected to wear depends upon the quality of the 
materials and the method of construction. For the 
heavier traffic of many of the smaller cities^ and 
streets of moderate traffic in the larger cities, brick has 
shown an endurance which indicates it to be a satis- 
factory and economical material. 

In contracting for the construction of brick pave- 
ments, many cities require the contractor to guarantee 
the pavement for a term of years, making all necessary 
repairs during the period of guaranty. This is intended 
as an assurance of the quality of the work. A guaranty 
of the pavement for one year may often be of use in 
discovering any serious defects in construction, and 
will not add materially to the cost, but the engineer 
in charge of the work has means of accurately judging 
its quality and, where a long period of maintenance is 
required, it is doubtful whether the gain in quality is 
sufficient to warrant the increase in price necessitated 
by the guaranty. 



CHAPTER IX. 
ASPHALT PAVEMENTS. 

Art. 66. Asphalt 

The term asphalt, as commonly used, includes all of 
the solid bitumens which are used in the construction of 
street pavements. Some account of the classification and 
characters of these bitumens has been given in Chapter 
VL The materials usually classed as asphalts include 
the true asphalts, gilsonite, and grahamite. Some residual 
pitches derived from the distillation of petroleums are 
also frequemtly called asphalts. In California, the 
residual pitches derived from asphaltic petroleums, as 
prepared for use in paving, are known as " D " grade 
aspJtalts, while in the East, residual pitches from some 
of the semi-asphaltic petroleums, used for road binders, 
are sometimes called oil asphalts. 

The natural asphalts used in paving in the United States 
are obtained mainly from Trinidad and Venezuela, 
although they are found to some extent in the United 
States, Cuba, and Mexico. The most important supplies 
of native solid bitumen in the United States are the 
gilsonites, which are being used to considerable extent 
for street pavements. Grahamite has also been used 
in a small way. 

TRINIDAD ASPHALT. 

The most important source of supply of asphalt for 
street pavements in the United States is that of the 

263 



264 A TEXT-BOOK ON ROADS AND PAVEMENTS. ■ 

island of Trinidad, W. I. This asphalt is known as 
lake asphalt or land asphalt, according to the source 
from which it is obtained. Lake asphalt is found in a 
large deposit known as the pitch lake. This lake covers 
an area of over 100 acres", and lies in a deep crater with 
steeply sloping sides. The pitch seems to be, or to have 
been, forced up from below, and it is more or less in 
motion, excavations in the surface being gradually filled 
by flow of material from sides and bottom. Upon 
exposure to the air, the pitch slowly hardens, is some- 
what softer near the center of the lake than at the sides, 
and it has been supposed that the supply from sub- 
terranean sources still continues to some extent. It has 
also been found that the surface of the lake is higher 
in the center than at the sides, and that the general 
elevation of the surface has been lowered somewhat by 
the large quantities of material which have been removed 
from it.* 

In Trinidad lake asphalt, the bitumen occurs mixed 
with considerable quantities of finely divided mineral 
matter, as well as with water and small amounts of other 
impurities. In order to remove this water and any 
vegetable impurities which the crude material may con- 
tain, the asphalt is refined by heating sufficiently to 
vaporize the water and melt the bitumen. This is accom- 
plished either by the use of a large kettle heated directly 
by fire, or by passing steam through pipes inside the tank 
containing the asphalt. During the heating the material 
is agitated by a current of air or steam. Wlien the water 
has been driven off, and the material is thoroughly melted, 
the liquid asphalt is drawn off and is known as refined 

* For a complete description of the Trinidad pitch deposits see the 
"Report of the Inspector of iVsphalts and Cements of the District of 
Columbia," for 1891-92. 



ASPHALT PAVEMENTS. 265 

asphalt. In refining the asphalt no effort is made to 
remove the mineral matter, which is present in a finely 
divided state, and is utilized to replace sand or dust 
which must otherwise be added in forming the paving 
mixture. 

Refined Trinidad Lake Asphalt consists ordinarily of 
about 54 per cent to 57 per cent bitumen, 5 per cent to 
8 per cent of organic matter not soluble in carbon bisul- 
phide, and 35 per cent to i^'i per cent of mineral matter. 
The bitumens contain about 63 per cent to 66 per cent 
of malthenes (according to Richardson's classification), 
the remainder being asphaltenes, with sometimes about 
I per cent of carbenes. These asphaltenes contain con- 
siderable sulphur and are hard, brittle substances, Avhich 
do not melt, but are readily soluble in the asphaltic oils. 
The non-bituminous organic matter is mainly material 
which seems to have been formed through oxidation of 
some of the harder bitumens of the asphalt. It contains 
a considerable amount of sulphur and may be considered, 
hke the finely divided mineral matter, as of use as filler. 
The mineral matter in Trinidad asphalt is found in a 
finely pulverized condition and quite uniformly distributed 
through the mass. 

Trinidad Land Asphalt is found in numerous deposits 
in vicinity of the pitch lake, most of them covered with 
soil. These deposits may have been formed either from 
the overflow of the lake or from independent sources, the 
action of which has long since ceased. The character 
of the land asphalt is more variable than that of the 
lake, and seems to depend upon the length of time it 
has been exposed to the weather. The bitumen of the 
asphalt undergoes a gradual hardening with time, the 
percentage of malthenes becoming less as compared with 
that of the asphaltenes. In some instances the amount 



266 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

of mineral matter is greater, while the non-bituminous 
organic matter is increased by the changing of some 
of the bitumen to an insoluble condition. 

Refined Trinidad Land Asphalt consists commonly of 
about 51 per cent to 55 per cent bitumen, 7 per cent 
to 10 per cent organic matter insoluble in carbon bisul- 
phide, and 37 per cent to 40 per cent of mineral matter. 
The bitumen contains from 50 per cent to 63 per cent 
of malthenes, soluble in 88° naphtha solution. In the 
use of the land asphalts, larger quantities of fluxing 
materials must be employed to bring the material to proper 
consistency for use, and, on account of the variable 
character of the asphalt, much care must be taken in 
handling it to secure good results. 

BERMUDEZ ASPHALT 

Asphalt obtained from the Bermudez pitch lake in 
Venezuela is commonly known as Bermudez asphalt. 
This deposit is much greater in area than that at Trinidad, 
being some 900 acres in extent. It is, however, shallow 
in depth; not, as in Trinidad, in a deep crater, but 
spread out in a flat layer over the surface of the ground 
from a number of springs, some of which are still active. 
The depth of the deposit is from two to nine feet and 
covered with vegetation. The ground is flat and swampy, 
so that excavations fill with water, and in the rainy 
season the deposits are largely covered with water. 

The asphalt has come from the spring in a soft condition, 
and afterward hardened upon exposure. Some of these 
springs are still active, and are surrounded by small 
areas of soft pitch raised above the general level of the 
deposit. The surface of the deposit in general is covered 
with a crust, probably due to coking produced by burning 



ASPH.\LT PAVEMENTS. 267 

vegetation, and this surface is cut through to obtain the 
asphalt from beneath. These deposits have been described* 
by Mr. Richardson, who has made a careful study of 
them. 

The Bermudez asphalt is more • variable in character 
than that from Trinidad. It contains very little mineral 
matter, and the bitumen is softer, carrying a higher per- 
centage of malthenes. The crude asphalt contains con- 
siderable water and organic impurities, which are removed 
by refining as with the Trinidad asphalts. 

Refined Bermudez Asphalt contains usually 93 per 
cent to 97 per cent bitumen, 2 per cent to 5 per cent 
of other organic matter, and i per cent to 3 per cent 
inorganic, or mineral matter. The bitumen contains 
64 per cent to 72 per cent of malthenes soluble in ^'&° 
naphtha solution, and has 2 per cent to 5 per cent of 
petrolenes, volatile at 325° F. in 7 hours. It is therefore 
somewhat softer and more volatile than the Trinidad 
asphalt. 

Another deposit of asphalt in Venezuela produces 
material known as " Maracaibo asphalt " somewhat 
similar to the Bermudez asphalt and used for paving. 

UTAH ASPHALT 

Considerable deposits of asphaltic materials are found 
in Utah and Colorado. Of these, the most important 
are large deposits of nearly pure bitumen known as 
Gilsonite. This material carries no mineral matter and 
is almost entirely soluble in carbon bisulphide. It does 
not therefore need refining or dehydrating before use. 
The proportion of malthenes is usually much less than 

*0n the Nature and Origin of Asphalt, Long Island City, 1898. 
The ^Modern Asphalt Pavement, New York, 1908. 



268 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

that of the Trinidad asphalts, although there is considerable 
variation in the composition of the different veins. The 
character of the hydrocarbons is also quite different 
from those of the asphalts, the malthenes being mainly 
composed of unsaturated hydrocarbons. The Utah gil- 
sonite is used to considerable extent for paving and also 
in the preparation of asphalt cement for filling the joints 
in brick and stone pavements, as well as asphalt paints 
and waterproofing mixtures. 

CALIFORNIA ASPHALT 

Several deposits of asphalt in Southern CaKfornia have 
been developed and more or less used for paving purposes. 
The individual deposits in most cases are small in extent 
and somewhat expensive to work. The asphalt usually 
differs from those already described in being harder, the 
bitumen containing less malthenes. Some of them have 
a higher percentage of mineral matter and less bitumen 
than the Trinidad; others have only a small amount 
of mineral matter but are composed of a harder bitumen. 

Bitumen similar to that in the natural asphalts is 
produced as a residual pitch in the distillation of the 
asphaltic petroleums of California. The character of this 
bitumen depends upon the extent to which the distillation 
is carried and the care used in the operation. These 
residues have been used to a considerable extent in 
paving under the name '' Z> " grade asphalt. With proper 
manipulation of the process the product may be con- 
trolled and the proportions of malthenes and asphaltenes 
regulated. Where the material is overheated and burned, 
some of the asphaltenes are changed to carbenes, or 
to insoluble organic matter. Much care is therefore 
required for the preparation of good materials, and 



ASPHALT PAVEMENTS. 269 

failures have frequently resulted from the use of that 
which has been carelessly prepared and cracked. It is 
necessary that the residuum be prepared at a low tem- 
perature to prevent cracking. The following specification 
is given* by Richardson for " D " grade asphalt suitable 
for use in paving: 

" ' D ' grade asphalt should be the residue from the 
careful distillation, with steam agitation, of some suitable 
Cahfornia petroleum at as low temperature as possible 
and certainly not exceeding 700° F. It shall be free 
from carbon or suspended insoluble matter, which are 
evidence of excessive cracking. 

" It shall be soluble to the extent of at least 98 per 
cent in carbon disulphide, 95 per cent in cold carbon 
tetrachloride and not less than 65 nor more than 80 per 
cent of it shall be soluble in 88° Pennsylvania naphtha, 
preferably nearer the former figure. 

" It shall not flash below 450° F. and shall have a 
density between 1.04 and 1.06. It shall not volatilize 
more than 8.0 per cent at 400° F. in 4 hours, and shall 
have a penetration between 40° and 70°. It shall melt 
at not less than 140° nor over 180° F. on mercury, according 
to the method in use in the New York Testing Laboratory, 
and shall yield not more than 15 per cent of fixed carbon 
on ignition. 

" It shall have a consistency of not less than four 
(4) mm. penetration at 78° F. when tested for five (5) 
seconds with a No. 2 needle weighted with 100 grams." . 

ASPHALTIC SAND. 

Deposits of sand impregnated with asphalt occur at a 
number of points in California, Kentucky, Utah, and 

* The Modern Asphalt Pavement, New York, 1908. 



270 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Indian Territory. These deposits consist of sand, or 
sandstone, saturated with bitumen. They differ from 
each other in the amount of bitumen found in them, 
and in the sand grains. They contain from about 5 
per cent to 20 per cent of bitumen, which in most instances 
have a larger percentage of makhenes and are much 
softer than those of the Trinidad and Bermudez asphalts. 
They also frequently contain considerable petrolene, or 
matter volatile at 325° F. in 7 hours; in some instances 
as much as 10 per cent to 12 per cent. The bitumens 
of these materials may be classed as malthas rather 
than asphalts, and contain too much volatile matter for 
successful use as paving materials. They have been 
used to some extent in paving. In California the bitumen 
has been extracted from the sand by the use of naphtha 
and then refined and used in the same way as the Venezuela 
asphalts. The Kentucky asphaltic sandstones have been 
used by adding to them a harder bitumen, and finer 
mineral matter for filler. The Indian Territory material 
has been used for pavements by mixing it with an asphaltic 
limestone which also occurs in the same locality. 

ROCK ASPHALT. 

Limestone impregnated with bitumen occurs in many 
places in Europe and a few localities in the United States. 
The rock is mined at a number of places in Europe, 
notably at Seyssel, France; Travers, Switzerland; Ragusa, 
Italy; and Verwohle, Germany. It is usually composed 
of nearly pure carbonate of lime, impregnated with from 
5 per cent to 20 per cent of bitumen. Natural rock 
asphalt suitable for paving purposes usually contains 
from 9 per cent to 20 per cent of bitumen. The rock 
should be of fine, even grain, and have the bitumen 



ASPHALT PAVEMENTS. 27T 

uniformly distributed through it. In forming the surface 
material for pavements, the rock from different mines 
is commonly mixed in such proportions as to give about 
10 per cent to 12 per cent of bitumen in the mixture. 

Deposits of bituminous limestone exist in Texas, 
Indian Territory, and Utah in the United States. That 
of the Indian Territory has, as already stated, been used 
for paving in connection with asphaltic sand. 

Art. 67. Asphaltic Cement. 

Refined asphalt is brittle at ordinary temperatures 
and possesses httle cementitious value. To bring it 
to a proper consistency it is heated to a temperature 
of about 300° F. and mixed with heavy bituminous 
oil, which serv^es as flux. The product is then known 
as asphalt cement. 

Fluxes. The material commonly used to soften asphalt 
in preparing paving cement is the oil residuum resulting 
from the distillation of petroleum. These residuums are 
prepared as mentioned in Art. 45, and consist almost 
entirely of malthenes from which the petrolenes and 
Hghter oils have been removed. 

In the preparation of the residuums for this purpose, 
it is necessary that the distillation be carried far enough 
to remove all oils which may be volatihzed at the tem- 
perature to which the material must be subjected in 
use, in order that the consistency of the cement remain 
constant. Beyond this point the extent to which the 
distillation should be carried depends upon the kind 
of asphalt and the ser\dce to which it is to be subjected. 
With dense and heavy oils a larger amount of oil must 
be used to reach the same consistency than with light 
oils, and the stabihty of the mixture is greater when 



272 A TEXT-BOOK ON ROADS AND PAVEMENTS. * 

maintained in heated condition for a long time, on account 
of the less volatile nature of the oils. The character of 
the flux employed must be such as to give a proper 
relation between the percentages of malthenes and asphal- 
tenes present in the resulting cement when brought to 
the required consistency. This is usually regulated in 
practice by specifying the penetration to be shown by 
the cement as well as the limits within which the per- 
centage of malthenes to total bitumen may vary. 

For Trinidad and Bermudez asphalts, fluxes made from 
either the parafline, or the semi-asphaltic, petroleums 
have been found quite satisfactory, although the asphaltic 
oil when carefully prepared seems the more desirable. 
The dense residuums of the Cahfornia asphaltic oils 
have not been found successful with these asphalts. The 
amount of residuum required for these asphalts varies 
from about 15 per cent to 25 per cent of the weight of 
the asphalt. If the residuum be so dense as to require 
larger quantities to flux the asphalt to the required con- 
sistency, the percentage of malthenes becomes too great 
and the cement is made more susceptible to temperature 
changes. 

Asphaltic materials hke gilsonite and grahamite con- 
taining less percentages of malthenes are fluxed by the 
use of heavy asphaltic residuums, which supply to the 
cement the lacking malthenes. Much larger proportions 
of flux must be used with these materials than are required 
for the Trinidad and Bermudez asphalts, the amount 
varying with the percentage of malthenes in the asphalt. 

Natural malthas have sometimes been used as fluxes 
for asphaltic cement, but in most instances difficulty 
has been met in their use due to the fact that they con- 
tain considerable of the lighter oils, petrolenes, which are 
volatilized at the temperature required for mixing, thus 



ASPHALT ^ PAVEMENTS. 273 

leaving the maltha too hard to act satisfactorily as a flux. 
The same difficulty is met in use of carelessly prepared 
petroluem residuum, which may lack uniformity of com- 
position, and contain both volatile oils and hard asphaltenes. 
Preparation of Cement. In preparing asphalt cement 
the asphalt is first melted and raised to a temperature, 
of about 300° F. The flux is then added at a tempera- 
ture of 150° to 200° F. The mass is then agitated with 
jets of steam or air. The agitation is continued from 
4 to 8 hours, or until the mass comes to a uniform and 
homogeneous condition. The refined asphalt cement is 
then drawn off. Careful and expert manipulation is 
necessary to secure a uniform product of proper con- 
sistency. Continued agitation with air causes hardening 
of some of the bitumens and volatihzes some of the 
lighter oils. 

Art. 68. Tests for Asphaltic Cement. 

For the purpose of controlhng the character of surface 
mixtures to be used upon asphalt pavements, tests are 
commonly made of the asphalt cement, as well as of the 
surface mixture itself. In testing asphalt cement the 
total amount of bitumen is usually determined; the 
hydrocarbons composing the bitumen are separated into 
their various classes, and the consistency of the mixture 
as well as the effect of temperature upon the consistency 
is examined. These tests are much the same as those 
given in Art. 48 for bitumen for road purposes, but vary 
in some particulars according to the use for which the 
material is intended. Tests for specific gravity, fixed 
carbon, and melting-point are there given and will not 
be repeated here. 

Total Bitumen. The total bitumen in asphalt, or 



274 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

asphalt cement, is determined by testing its solubility 
in carbon disulphide. The following method is recom- 
mended as standard by the " Committee on Standard 
Test for Road Materials," of the American Society for 
Testing Materials.* 

" It was decided, owing to the great variety of con- 
ditions met in asphalt and like bitumen, that it was 
impossible to specify any one method of drying that 
could be at all satisfactorily apphed in every case; it 
is therefore supposed that the material for analysis has 
been previously dried either in the laboratory or in the 
process of refining or manufacture, and that water, if 
present, exists only as moisture in the hydroscopic form. 

" The material to be analyzed, if sufficiently hard and 
brittle, is ground and then spread in a thin layer in 
a suitable dish (iron or nickel will do), and kept at a 
temperature of 125° C. for one hour. In the case of 
paving mixture, where it is not desirable to crush the 
sand grains, a lump may be placed in the drying oven 
until it is thoroughly heated through, when it can be 
crushed down into a thin layer and dried as above. If 
the material under examination contains any hydro- 
carbons at all volatile at this temperature, it will of 
course be necessary to resort to other means of drying. 
Tar or oils may be dehydrated by distillation and the 
water-free distillate returned to the residue and thor- 
oughly incorporated with it. 

" Analysis of Sample. After drying, from 2 to 15 
grams (as may be necessary to insure the presence of 
I to 2 grams of pure bitumen) is weighed into a 150-c.c. 
tared Erlenmeyer flask, and treated with 100 c.c. of 
carbon disulphide. The flask is then loosely corked 
and shaken from time to time until ah large particles 

* Proceedings, American Society for Testing Materials, Vol. VI, 1906. 



ASPHALT PAVEMENTS. 275 

of the material have been broken up. It is then set 
aside for 48 hours to settle. The solution is decanted 
into a similar flask that has been previously weighed. 
As much of the solvent is poured off as possible without 
disturbing the residue. The contents of the first flask 
are again treated with fresh carbon disulphide, shaken as 
before, and then put away with the second flask for 
48 hours to settle. 

" The hquid in the second flask is then carefully decanted 
upon a weighed Gooch crucible, 3.2 cm. in diameter at 
the bottom, fitted with an asbestos filter, and the contents 
of the first flask are similarly treated. The asbestos 
filter is made of ignited long-fiber amphibole, packed 
in the bottom of a Gooch crucible to the depth of not 
over -J inch. In filtering no vacuum is to be used and 
the temperature is to be kept between 20° C. and 25° C. 
After passing the liquid contents of both flasks through 
the filter, the residue on the filter is thoroughly washed, 
and the residues remaining in them are shaken with 
more fresh carbon disulphide and allowed to settle for 
24 hours, or until it is seen that a good subsidation has 
taken place. The solvent in both flasks is then again 
decanted through the filter and the residues remaining 
in them are washed until the washings are practically 
colorless. All washings are to be passed through the 
Gooch crucible. 

" The crucible and both flasks are then dried at 125° C. 
and weighed. The filtrate containing the bitumen is 
evaporated, the bituminous residue burned, and the weight 
of the ash thus obtained added to that of the residue 
in the two flasks and the crucible. The sum of these 
weights deducted from the weight of substance taken 
gives the weight of soluble bitumen." 

Bitumen Soluble in Naphtha. This test is employed 



276 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

for the purpose of determining the relative amounts of 
asphaltenes and malthenes present in the bitumen. 
When used in specifications it is designed to insure a 
proper relation between these classes of hydrocarbons, 
the object being to avoid materials containing too great 
percentage of asphaltenes, and the use of light oils as 
fluxes. The Committee of the American Society for 
Testing Materials recommeud that the same method 
be employed as for obtaining the total bitumen. They 
also recommend that the naphtha used be described 
by giving the temperatures between which it distills and 
its specific gravity. Naphtha of a density of 88° Be. 
at 60° F. is commonly employed for this purpose. 

Bitumen Soluhle in Carbon Tetrachloride. Tests of the 
solubility of solid bitumens in carbon tetrachloride are 
made for the purpose of determining the character of 
the bitumen through separating the carbenes from the 
asphaltenes. This test is commonly made in the same 
manner as is used in determining the solubiHty in carbon 
disulphide. The following method is given by the Asso- 
ciation for Standardizing Paving Specifications, in 191 1: 

" Weigh off one gram of material, cover with 200 c.c. 
carbon tetrachloride, in Erlenmeyer flask, about 4:00 
in the afternoon. Allow to stand over night in dark 
cupboard. Next morning, at 10 o'clock, Gooch crucible 
with felt is weighed and solution poured through. Wash 
with carbon tetrachloride, dry, at 100° C. and weigh." 

Heat Test. This test is employed to determine the 
amount of the lighter and more volatile hydrocarbons 
in the bitumen. It separates the petrolenes from the 
heavier hydrocarbons (malthenes and asphaltenes). The 
test is made by determining the loss in weight suffered 
by the material upon being heated for a definite time 
at a constant temperature. It is designed to show whether 



ASPHALT PAVEMENTS, 277 

the asphalt cement will be materially changed by heat 
in forming the surface mixture. 

Methods of making the test are given in Art. 48. 

Penetration Test. The consistency of asphalt cement 
is determined by measuring the penetration of a No. 2 
needle under a standard weight (usually 100 grams), 
in a given interval of time (commonly 5 seconds). The 
tests must be made at a standard temperature (usually 
77° F.). Machines for making this test have been 
devised by Mr. A. W. Dow* and by Clifford Richardson.f 

The Dow apparatus consists of a No. 2 needle in- 
serted in a short brass rod which is held in an aluminum 
rod by a binding screw\ The aluminum rod is secured 
in a framework so balanced that when it is supported on 
the point of the needle the framework and rod will 
stand in an upright position, allowing the needle to 
penetrate perpendicularly without the aid of support. 
The frame, aluminum rod, and needle weigh 50 grams ; 
additional weight, when desired, is placed on the bot- 
tom of the frame. The motion of the sliding part is 
communicated by a thread to an index arm moving 
over a graduated disk. 

"To make the penetration test, the samples of 
asphalt cement contained in circular tins, along with 
the glass dish, are placed in a receptacle containing 
at least 5 inches of water, which should have been pre- 
viously brought to the temperature at which it is 
desired to make the test. While the samples are under 
the water it should be stirred every few minutes, best 
with a thermometer, and the temperature kept con- 
stant w^hen necessary by the addition of hot or cold 
water as the case may require. The samples should 

* Proceedings, American Society for Testing Materials, Vol. III. 
t Proceedings, American Society for Testing Materials, Vol. VII. 



27« A TEXT-BOOK ON ROADS AND PAVEMENTS. 

remain under water at least 15 minutes, and in cases 
where their temperature is not near that at which 
the test is made they should be left in possibly half an 
hour. After the samples have remained in the water 
a sufficient time to have attained its temperature they 
are ready to be penetrated.'" 

For the purpose of determining the effect upon con- 
sistency of changes of temperature, tests are made of 
the penetration at different temperatures. Mr. Dow 
recommends the following standards: ''The needle 
which I have adopted as a standard for penetration 
is a No. 2, manufactured by R. J. Roberts, Redditch, 
England. All the needles, however, obtained in a 
package cannot be used for penetrating, as they vary 
somewhat in shape, and only those are selected which 
give a penetration corresponding to the standard 
needle. The standards that I have adopted for this 
machine are: At 32° F. or lower, the distance in one- 
hundredths of a centimeter that a No. 2 needle will 
penetrate into the sample in one minute of time w^hen 
weighted with 200 grams. For tests made at a tem- 
perature of 77° F., the distance in one-hundredths of 
a centimeter that a No. 2 needle will penetrate into 
the sample in 5 seconds of time when weighted with 
100 grams. For tests made at a temperature of 100° F., 
or above, the distance in one-hundredths of a centi- 
meter that a No. 2 needle will penetrate in 5 seconds of 
time weighted with 50 grams. 

" The following is a table giving the penetration and 
ductility of three classes of asphalt cement, which I 
have designated as A, B, and C: 



ASPHALT PAVEMENTS. 



279 



Penetration at — 

32° F 

77° F 

100 F . 

115° F 

Ductility at 77° F 



A 


B 


10 


13 


55 


47 


150 


no 


350 


220 


300 


75 



25 

45 

75 

120 

20 



" It has been found from practical experience that 
it is not safe to use an asphalt that is more susceptible 
to changes in temperature than sample A, given in 
the table, for if it were more susceptible than this, 
and made to a softness to give sufficient ductility at 
low temperatures, it would be too soft for use at high 
temperatures. The average paving cement gives 
penetrations such as represented by B in the table. 
Sample C in the table represents the least susceptible 
cement which I have found on the market. This non- 
susceptibility to change in temperature would be of 
great advantage if it were not for the fact that the 
cement is lacking in ductility. There is a law which 
I have found that invariably applies to the proper- 
ties of asphalt cements, that is, that the less susceptible 
cement is to change in temperature, the less ductile it 
is at normal temperatures, and inversely, the more 
susceptible the more ductile is the cement.'' 

Test for Ductility. Mr. A. W. Dow * has proposed 
a test of the ductility of asphalt cement by determining 
the distance in centimeters that a prism of cement can 
be drawn out before breaking. The prism he uses is 
5 centimeters in length with a square cross-section of 
I centimeter. The test piece is molded with the ends 
in clips, which may be attached to apparatus for 



* Proceedings, American Society for Testing Materials, Vol. III. 



28o A TEXT-BOOK ON ROADS AND PAVEMENTS. 

applying the pull. The clips are pulled apart at a 
speed of I centimeter per minute, while immersed in 
water at the required temperature. ''Sufficient work 
has not been done on the ductility test at low temper- 
atures to be able to state any standard at the present 
time, but it has been found that it is not safe for an 
asphalt having a consistency of 40 penetration at 77° F. 
to pull less than 20 centimeters at this temperature in 
the above ductility test.'' 

Impact Test. An impact test for the purpose of 
determining the toughness of asphalt surface mixtures 
has been proposed by Messrs. Richardson and Forrest.* 
''The test pieces were made as follows: The surface 
mixture was brought to such a temperature as would 
be found necessary in handling it upon the street, a 
weighed amount, such as has been found by experi- 
ence would yield a cylinder after compression of I 
inch in height, is placed in a C3dindrical mold, closely 
resembling the ordinary diamond mortar of the labo- 
ratory, of a diameter of I J inch. The mold is supported 
on the rigid block of timber I if by 9^ inches square 
by Z^^ inches high. The warm steel plunger is placed 
upon the top of the hot mixture, above which is a 
cylinder of steel weighing 10 pounds, running in 
grooved guides, which can be allowed to fall upon it 
from a height of 3 feet. After a few gentle taps to seat 
the plunger, the weight is raised and allowed to fall 
freely 10 times. The cylindrical mold is then inverted 
and the plunger introduced at the other end in a space 
left for this purpose by a boss on the base supporting 
the mold. Ten additional blows are then given on 
this end of the cylinder. In this way it has been found 
that satisfactory and uniform compression is obtained. 

* Proceedings, American Society'for Testing Materials, Vol. V. 



ASPHALT PAVEMENTS. 2»l 

The cylinders are then weighted to determine if the 
density is satisfactory, and measured to see that they 
are of uniform height, I inch or nearly so. On cooling 
they are ready to be tested, in the same manner 
employed by Mr. Page for rock cylinders, at whatever 
temperature may be selected" (see Art. 37). 

Separation of Bitumen. For the purpose of testing 
the bitumen in surface mixtures, or in asphalt cement 
containing considerable mineral matter, it may be 
necessary to separate the bitumen from the mixture. 
The following method is given by Mr. Dow: * ''The 
pure bitumen is obtained from an asphalt, or asphaltic 
cement, by extracting with carbon disulphide and 
evaporating off the solvent. The procedure that I 
have found to give the best results is as follows: Suffi- 
cient of the asphalt or asphaltic cement to give 30 
grams of pure bitumen is placed in a large Erlenmeyer 
flask. Between 300 and 400 centimeters of carbon 
disulphide is added, the flask corked and then shaken 
from time to time until none of the asphalt is seen 
adhering to the sides or bottom, after which the flask 
is set aside and allowed to stand for 24 hours. The 
carbon disulphide is then decanted off carefully from 
the residue into a second flask. The residue is again 
treated with 200 or 300 cubic centimeters of the 
solvent and shaken as before. After the solutions 
in the two flasks have been allowed to subside for 24 
hours, the contents are carefully decanted off on to an 
asbestos filter, passing the contents of the second 
flask through the filter first. The solvent containing 
the bitumen is then distilled in a flask until just suffi- 
cient remains to have the contents liquid. It is then 
poured into a flat evaporating dish and further heated 

* Proceedings, American Society for Testing Materials, Vol. III. 



282 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

on the steam-bath, stirring from time to time, until 
the greater part of the carbon disulphide is evaporated. 
About one-half cubic centimeter of water is next 
incorporated into the residue of bitumen and the heat- 
ing continued over a burner until all foaming ceases, 
after which it is kept at 300° F. for 10 minutes. 
While heating over the burner the bitumen should be 
stirred constantly with a thermometer and care exer- 
cised that the temperature is kept constant at 300° F„ 
It is doubtful whether in all cases the last traces of 
carbon disulphide are removed, even by this method, 
and it is also likely that the bitumen obtained in this 
way is often slightly harder than that contained in 
the original asphalt or cement; but its physical prop- 
erties, as far as ductility and susceptibility to change 
in temperature go, will be relatively the same, and a 
sufficiently close approximation can be made of the 
consistency of the bitumen in the original sample to 
answer all practical purposes. As the removal of the 
last traces of carbon disulphide is very difficult, and a 
soft bitumen is liable to be hardened in so doing, I 
make it a practice, wherever it is possible, to extract 
the bitumen from an asphalt before it has been soft- 
ened into the paving cement. In this way I find it 
easier to remove the last traces of solvent from this 
hard bitumen, and at the same time with relativel}^ 
less hardening. This bitumen from the asphalt is 
then fluxed into a paving cement b}^ adding to it an 
amount of flux equivalent to that used in making the 
paving cement from the asphalt. It is fortunate that 
nearly all the asphalts met with in commerce that are 
not pure bitumen are of a hard nature, so that the 
above method is applicable in practically^ all cases. 
This of course does not apply to bituminous rock, and 



ASPH.\LT PAVEiMENTS. 



283 



the only way possible to estimate their quality is by 
examining the extracted bitumen, which is done as 
just described. It is well to note here that in cases 
where the bitumen hardens materialh^ in the removal 
of the solvent, such a bitumen will be rejected b}^ 
hardening too much in the heat test." 

Examination of Mineral Aggregates. For the purpose 
of determining the character of the mineral aggregate 
used in a surface mixture, or present in an asphaltic 
cement, the American Society for Testing Materials 
recommends* that it be passed through sieves of the 
following sizes in the order named. 



Meshes per 
Linear Inch. 



Diameter of Wire. 



Inches. 



Mm. 



200 

100 

80 

50 

40 

30 
20 
10 



0.00235 
o . 0045 
o- 00575 
0.009 
0.01025 

0.01375 

0.0165 
0.027 



0.05969 

O.I 143 
o. 1460 
o. 22865 

0.26035 
0.34925 

0.4I9I 



Tests Required by Specifications. Many tests have been 
proposed for the control of asphalt paving mixtures, or 
used in the study of asphalt materials, by various inves- 
tigators. In general, however, specifications used by 
municipal engineers have depended upon a contractor's 
guaranty for the character of the work rather than upon 
inspection of the materials and workmanship. This has 
not proven altogether satisfactory and it is now common 
to have definite specifications for the materials employed 
and subject them to test in the city laboratories. 



284 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Art. 69. Surface Mixtures. 

The material commonly employed for the surfaces 
of asphalt pavements consists of a mixture of asphalt 
cement, powdered limestone, and sand. The mixtures 
used in different places have varied considerably in 
character, according to the nature of the materials 
available and the amount and consistency of the 
bitumen employed. Experience has gradually devel- 
oped the practice in different places, the work at first 
being largely experimental, defects in the early work 
being corrected by modifications in later mixtures. 
Much of this work has been done by very haphazard 
methods and without any careful analysis of the 
causes of defects and failures, or of the differences in 
materials used in different places. 

Sand. It is common to grade sand in size by 
sifting through sieves of 10, 20, 30, 40, 50, 80, lOO and 
200 meshes to the linear inch, finding the percentage 
which passes each sieve and is caught by the next 
finer one. The portion which passes the 200-mesh 
sieve is too fine to be considered as sand, and is classed 
with the stone powder which is added as filler. The 
sands used for asphalt surface mixtures are much 
finer than those employed in cement mortars. In 
sand for this purpose most of the sand is usualh^ fine 
enough to pass the 40-mesh sieve. In some instances, 
the bulk of the sand will pass through the 80 and 100- 
mesh sieves; in others, the larger portion will only pass 
the 40 and 50-mesh sieves. A grading of sizes in the 
sand is desirable on account of reducing the amount 
of voids to be filled by the asphalt, and frequently 
when a natural sand of correct sizes is not available, 
it is possible to secure a proper relation of sizes of 



ASPHALT PAVEMENTS. 285 

grain by mixing two or more sands of differing sizes. 
In general, sand so graduated as to leave a small 
percentage of voids is desirable in order that the 
interstices may be fully filled with bitumen, but the 
sand of greatest density is not necessarily the best 
for this purpose, as there may be instances where the 
percentage of voids would not admit of a sufficient 
amount of bitumen. 

The voids in the sand are commonly tested by 
filling a measure w^ith packed sand and then deter- 
mining the quantity of water that can be added to it. 
A better method with fine material is to determine 
the specific gravity of the sand and also the weight of 
a given volume of it, the volume of voids being the 
difference between the measured volume and the 
volume of solid matter represented by its weight. 

The sand used should be hard and tough in order to 
resist wear well, but its chemical character is not of 
special importance, although there seems to be a 
difference in the adherence of bitumen to the surfaces 
of different sands. The reason for this difference is 
not apparent and cannot be judged in advance of actual 
trial. The shape of grain does not usually appear 
important. Rounded grains often pack more easily 
and form a more dense mass, but it has been sometimes 
thought that they move more readily upon each other 
and form a less firm surface. 

Filler. The filler used in asphalt paving mixtures 
consists of very finelj^ ground mineral matter mixed 
with the bitumen for the purpose of rendering the 
surface more dense, and giving stiffness to it. The 
material commonly used for this purpose is ground 
limestone, although a number of other materials have 
been emplo^^ed. Ground clay may make a good 



286 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

filler; slaked lime, and Portland or natural cement 
are also used, especially good results being obtained 
with Portland cement. 

The filler should be finely ground; nearly all of it is 
usually required to pass a 200-mesh sieve, but the 
finer portions are too fine to be graded by the use of 
sieves. For this purpose the method of elutriation 
may be employed. By this method the powders of 
different degrees of pulverization are separated by 
observing the times required to settle after being shaken 
in a vessel of distilled water, the portions which settle 
in 15 seconds, one minute, and 30 minutes being deter- 
mined, and the relative fineness of different samples 
thus compared. 

Composition of Mixture. The relative amounts of 
asphalt cement filler, and sand required for a surface 
mixture must of course depend upon the properties of 
these materials. The fineness of the sand and of the 
filler, and the amount of mineral matter in the cement, 
are all important in determining the proper propor- 
tions. The proportions of materials are determined 
by weight, the purpose being to secure a proper 
amount of bitumen and of dust, as compared with 
the sand in the resulting mixture. The amount of 
bitumen required varies from about 9 per cent to 13 
per cent, most commonly between 10 per cent and 1 1 
per cent. Mr. Richardson gives •' the following mixture 
as a standard to be used for surfaces of Trinidad 
asphalt pavements: 

* The Modern Asphale Pavement, N. Y., 1905. 



ASPHALT PAVEMENTS. 



287 





Surface Mixture 


Sand. 


Bitumen . . 


Per cent. 
10.5 

13.0 
13.0 
13.0 

23-5 
II .0 

8.0 

5-0 
30 


Per cent. 


Passing — 

200-mesh sieve 




loo-mesh sieve . 


17.0 
17.0 
30.0 
13.0 


80-mesh sieve 


50-mesh sieve 




20-mesh sieve 


8 


lo-mesh sieve 


5-0 




100 .0 


100 .0 



This he regards as an exceptional!}^ good mixture. 
As the sands used in practice vary in the proportion 
of fine grain which they contain, the amount of 
bitumen must be correspondingly varied. When a 
larger portion of the sand passes the 80 and lOO-mesh 
sieves, a larger amount of bitumen and of filler may be 
introduced. When the sand is coarser, a smaller 
amount of bitumen is necessary in order that the 
pavement may not be soft enough to mark under the 
horses' feet. In sand which lacks the finer grains 
the percentage of bitumen which can be used without 
marking is often so low as to leave the material too 
porous and liable to the action of water. It is desir- 
able that the mixture contain all the bitumen that it 
will carry by the addition of filler without becoming 
too soft. A lack of bitumen may cause cracking of 
the surface. The quantity of bitumen required is 
also somewhat affected by the character of the sand 
grains and the extent to which the bitumen may adhere 
to and coat the grains. Some sands will ''take'" 
more bitumen than others of the same grading of sizes 



2SS> A TEXT-BOOK ON ROADS AND PAVEMENTS. 

without leaving a surplus of bitumen to render the 
material too soft. 

The amount of bitumen to be used in a surface mix- 
ture is commonly tested by the pat test. This consists 
in pressing a pat of the surface material in a piece of 
brown manila paper and observing the stain left upon 
the paper; the depth of the stain indicates to the 
experienced eye whether the right amount of bitumen 
has been used and whether the mixture has been prop- 
erly prepared. An impact test is also sometimes made 
to determine the resistance of the surface material to 
marking, and frequent analyses are made to tfest the 
correctness of the mixture. 

The traffic to which a street is subjected has much 
to do with the consistency required in the surface 
mixture. For streets of light traffic a softer mixture 
should be employed than for one with heavy traffic. 
The rolling out and working of the surface by heavy 
traffic will admit of a hard surface material which might 
crack under light traffic. The surface mixtures must 
in every case be suited to the local conditions of traffic 
and weather, that it may neither mark under the 
impact of traffic nor crack from shrinkage in cold 
weather. 

Method of Mixing. In the preparation of the surface 
mixture, the sand and asphalt cement are heated 
separately and then mixed while hot. Wlien two or 
more sands are used to obtain the proper grading of 
sizes, this mixing must first be accomplished, and great 
care is necessary in handling the sand in mixer and 
heater to prevent the segregation of sizes and bring the 
sand in uniform mixture at proper temperature (about 
330° to 350° F.) to the final mixture. The asphalt 
cement is also heated in a large heater where it is 



ASPHALT PAVEMENTS. 289 

agitated by steam jets to maintain the uniformity of 
mixture. 

The surface mixture is prepared in a mixer of small 
size which mixes 10 to 15 cubic feet at one operation, 
and is so arranged as to load directly into the wagon 
which takes it to the street. The mixing is accom- 
phshed by blades revolving on shafts in the mixing 
tanks, requiring about one to two minutes to make a 
complete mixture. The proportioning of the ingredi- 
ents is accomplished by weighing the proper quantity 
of each of the materials for a batch; the sand and filler 
are first introduced and mixed dry, and the asphalt 
cement then added and the whole mixed together. 
The mixture is then carried to the street at a tempera- 
ture above 300° F. 

Specification Requirements. The Association for Stand- 
ardizing Paving Specifications has recommended the 
following specification for the composition of surface 
mixture for an asphalt pavement: 

" The surface mixture shall consist of asphaltic cement, 

Portland cement (or stone dust), and sand proportioned 

by weight so that the resulting mixture will contain average 

proportions of the whole mixture as follows : 

Mixture A. Per Cent. 

Bitumen soluble in cold carbon disulphide. . . 11.01013.5 

Portland cement passing a No. 200 sieve .... lo.otois.o 

Sand passing a No. 80 sieve 18 . o to 36 . o 

Sand passing a No. 40 sieve 20.0 to 50.0 

Sand passing a No. 10 sieve 8 . o to 25 . o 

Sand passing a No. 4 sieve up to lo.o 

AIlXTURE B. 

Bitumen soluble in cold carbon disulphide. . , 10. 5 to 13 . 5 

Stone dust passing a No. 200 sieve 10. o to 15 .0 

Sand passing a No. 80 sieve 18.0 to 36.0 

Sand passing a No. 40 sieve 20.0 to 50.0 

Sand passing a No. 10 sieve ■ 8 . o to 25 . o 

Sand passing a No, 4 sieve up to lo.o 

Sieves to be used in the order named. 



290 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

" The item designated ' Portland cement (stone dust) 
passing a No. 200 sieve ' within the limits named herein 
includes in addition to the Portland cement (stone dust) 
fine sand passing a No. 200 sieve not exceeding 4§ per 
cent of the total mixture, and such 200-mesh mineral 
dust naturally self-contained in the refined asphalt. 

" Sand and asphaltic cement shall be heated separately 
to about 300° F. The maximum temperature of the 
sand at the mixers shall in no case be in excess of 375° 
F. at the discharge pipe. The Portland cement (or stone 
dust) shall be mixed with the hot sand in the required 
proportions, and then these shall be mixed for at least 
one minute with the asphaltic cement at the required 
temperature, and in the proper proportions in a suitable 
apparatus so as to effect a thoroughly homogeneous 
mixture. 

" The proportions of asphaltic cement shall at all 
times be determined by actual wieghing with scales 
attached to the asphaltic cement bucket. 

" The Portland cement (or stone dust) and sand must 
also be weighed unless a method of gauging approved 
by the (authorized city official) shall be used. 

" The contractor shall furnish every facility for the 
verification of all scales or measures. 

" The sand gradings and bitumen may be varied within 
the limits designated, in the discretion of (proper city 
official)." 

Rock Asphalt. The preparation of surface material 
with rock asphalt consists only in crushing and grinding 
the rock to powder, and heating the powder to drive 
off the water and soften the bitumen, so that it may 
be compacted in the pavement. The powder is heated 
to a temperature of 200° to 300° F. and is applied hot 
in laying the surface. 



ASPH.\LT PAVEMENTS. 29 1 

Rock asphalt as it occurs in nature varies widely in 
character, and seldom has a proper composition for use 
in pavements without admixture of other materials. In 
determining the suitability of a material of this class for 
use, the character as well as the quantity of bitumen 
contained by it must be considered, and its deficiencies 
supplied in the materials added to it. This may some- 
times be accomplished by mixing different grades of the 
asphaltic rock, or in other instances by adding other 
bitumens, or mineral matter. 

In determing a mixture of asphalt rock, as in the case 
of other asphalts, the local conditions of climate and 
trafhc must be considered and the quantity of bitumen 
be so proportioned as to remain solid in summer and 
not become brittle and lose cohesion in winter. The 
surface mixture for a rock asphalt pavement should 
ordinarily contain from 9 to 12 per cent bitumen The 
character of bitumen required is about the same as for 
a pavement made by mixing other asphalts with mineral 
matter. 



Art. 70. Construction of Sheet Asphalt Pavements. 

The work to be performed in laying a sheet asphalt 
pavement consists in grading and rolling the road-bed, 
placing the foundation, laying a binder course of bitu- 
minous concrete, and distributing and rolling the surface 
material so as to form a smooth surface. Sometimes the 
binder course is omitted, a thicker surface layer being 
employed to give sufficient stiffness and prevent the sur- 
face scaling from the foundation. 



292 A TEXT-BOOK ON ROADS AND PAVEMENTS. 



FOUNDATION. 

Concrete base. As a sheet asphalt surface has no 
power to sustain loads, acting only as a wearing sur- 
face, which must be held in place from below, it is 
essential that it be placed upon a very firm, unyielding 
foundation. It is consequently nearly always placed 
upon a concrete base, which is commonly formed of 
hydraulic cement mortar and broken stone, prepared 
as described in Art. 57. In the use of this base, it is 
necessary that the mortar be fully set, and the concrete 
thoroughly dry before the asphalt is laid upon it, as 
the placing of the hot surface material upon a damp 
foundation will cause the blistering and possible dis- 
integration of the surface by the steam generated from 
the base by the heat of the material. 

For moderate or heavy traffic in cities, the concrete 
base is commonly made 6 inches thick. For lighter 
traffic a less depth, 4 inches or 5 inches, is sometimes 
employed. The depth necessary will depend upon the 
nature of the road-bed as well as the weight of the 
traffic. It should be greater as the subsoil is less firm 
and well drained. 

Bituminous base. Sometimes a base has been used 
consisting of a layer of broken stone four or six inches 
thick rolled into place and coated with asphalt or coal 
tar paving cement. This is known as a bituminous 
base. The advantage which has been claimed for it is 
that the foundation and surface material become 
joined into a single mass, with the effect of anchoring 
the surface and preventing the formation of weathering 
cracks and wave surfaces, which are sometimes found 
when the hydraulic base and light surface layer are 
employed. The hydraulic base is commonly preferred 



ASPHALT PAVEMENTS. 293 

to the bituminous base, which is practically obsolete, 
because it forms an unyielding structure, not likely to 
be forced out of place by the weight of traffic at any 
point where the support of the road-bed may be 
weakened. 

Macadam base. In surfacing streets with asphalt 
which have previousl}^ been macadamized, it is some- 
times possible to use the old macadam as a base for the 
asphalt. This offers a good base in so far as it can be 
used without disturbance. It is difficult, however, to 
change the grade or reduce the crown without destroy- 
ing the bond of the macadam. Old brick and stone 
pavements may also be used in the same way^ where 
they can be used without disturbing them. 

BINDER COURSE. 

An intermediate layer known as the binder course is 
now commonly placed between the base and surface 
laj^er. This layer is ordinarily about i^ inches thick 
and consists of broken stone, which passes through a 
I inch screen, mixed with sufficient bitumen to thor- 
oughly coat the pieces of stone. The paving cement 
used in making the binder course should be of softer 
consistency than that used in the surface material, 
about 3 per cent of bitumen being usually required. 
The materials are mixed hot, laid and rolled in the 
same manner as the surface layer. This binder becomes 
consolidated with and gives added depth and strength 
to the surface, thus preventing the cracks and wave 
surfaces which maj^ otherwise appear. The binder, as 
commonly formed of broken stone, is open and porous, 
but in some instances stone of graded sizes and sand 
are employed to make a dense bituminous concrete. 



294 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

This is desirable practice, adding materially to the 
strength of the pavement under heavy traffic. It 
requires a larger amount of bitumen (about 5 per cent 
to 6 per cent) on account of the larger surface area of 
grains to be coated. 

The binder course has, in some instances, been 
replaced by a coatmg of asphalt paint, consisting of 
asphalt cement dissolved in benzene. The surface of 
the hydraulic base is painted with this mixture, which 
serves to cement the base to the surface layer. 

After the completion of the hydraulic base and when 
it has stood a sufficient length of time to harden and 
dry out, the binder course is placed and compacted. 
The binder is spread to uniform thickness over the 
base by use of shovels, all of the material being shoveled 
over m order to secure uniform compactness. It is 
then smoothed with rakes having long tines, and after 
partially cooling rolled with a 5 or 6 ton roller. 

SURFACE COURSE. 

Transportation. The materials for the binder and 
surface of asphalt pavements must be carried from the 
mixing plant to the street in some form of truck or 
wagon which will admit of the materials being delivered 
with small loss of temperature. Some form of dump 
wagon is commonly employed for this purpose, carrying 
from 2 to 4 tons of the materials at a load. The loss 
of heat is not rapid when the material is carefully 
handled and properly protected by tarpaulins, and the 
temperature of the mass should not be reduced more 
than about 10 degrees, where transportation to the 
street takes 2 or 3 hours. 

Placing. As soon as the rolling of the binder course 



ASPHALT PAVEMENTS. 295 

has been completed, it is ready for the surface layer. 
This is usually ij to i^ inches thick where a binder 
course is used, or 2 to 2^ inches in single course work. 
The surface material is distributed by hot shovels from, 
the piles into which it is dumped from the wagons, all 
the material being handled over as in the case of the 
binder. It is then spread into a smooth layer of proper 
thickness, with hot rakes, all lumps being broken and 
the material loosened up so that under the roller it 
may compact to a miiform densit}^ After raking 
smooth, the surface is rolled with a steam roller. A 
light roller (2 to 4 tons) is commonly used for the first 
rolling until the material is sufhcienth- compact to 
bear the heavier one (usually weighing 6 to 8 tons), 
w^hich completes the shaping of the pavement. A 
coating of dust, usualh^ hj^draulic cement, is given to 
the surface before the final rolling. This gives proper 
color to the surface. 

The handling of the material necessarily varies some- 
what with its character and requires, for good results, 
skill and experience on the part of the men in charge 
of the work. It is highly important that the material 
be so evenly distributed as to give a surface of uniform 
density; otherwise the surface may compress unequally 
under the traffic, becoming uneven and wavy. It is 
also necessary that the roUing be carefully done in 
order to properly compress the asphalt and bring the 
surface to the required form. When the surface is 
rolled out of shape through careless handling, it is 
difficult to bring it back again. The roller must be so 
balanced as to distribute the weight uniformly, a 
pressure of 200 to 300 pounds per linear inch of tire 
being required for the ultimate compression of the 
asphalt surface. 



296 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Rock asphalt. Pavements of rock asphalt are con- 
structed in the same manner as those from free bitumen. 
The rock asphalt makes a harder surface and is more 
slippery than that made from free bitumen. It has 
never come so extensively into use in the United 
States. In Europe, where rock asphalt is very exten- 
sively used, pavements made from free bitumens mixed 
with sand are frequently denominated artificial asphalt 
as distinguished from asphalt or natural asphalt, by 
which is meant the rock found impregnated with 
bitumen. 

In the European rock asphalt pavements the binder 
course is not so commonly employed as in the United 
States, and in many cases the finishing of pavement is 
by means of tampers and smoothing irons instead of 
rollers, the compression given to the surface not being 
so great, ultimate compacting being accomplished by 
the traffic. At the edges of the pavement and in places 
which cannot be reached by the roller, small hand tools 
such as hot smoothing irons and tampers are employed 
for finishing the surface. Sometimes also where the 
rolling has failed to compress the pavement into proper 
surface, it may be necessary to soften the surface with 
smoothing irons in order to reduce it to the required 
form. 

Art. 71. Asphalt Blocks. 

Asphalt paving blocks are frequently formed of a 
mixture of asphalt cement and crushed stone. The 
stone used is mainly trap, or granite, broken so as to 
pass a I inch screen. In early work, limestone was 
used; this was found to lack durability on account of 
the softness of the stone. The mixture is similar to 



ASPHALT PAVEMENTS. 297 

that used for the surface of a sheet pavement, containing 
about 8 per cent to 11 per cent of asphalt cement, 
7 per cent to 10 per cent limestone dust, and crushed 
stone 80 per cent to 85 per cent. 

The materials are heated to a temperature of about 
300° F., and mixed while hot in an apparatus arranged 
to secure the even distribution of the ingredients 
through the mass. The thorough incorporation of the 
various materials in the mixture is of first importance 
in producing homogeneous and uniform blocks, while 
the quality of the materials used needs as careful 
inspection as in the case of the surface material for 
sheet pavements. 

When the mixing is complete, the material is placed 
in moulds and subjected to heavj^ pressure, after which 
the blocks are cooled suddenly by plunging into cold 
water. 

These blocks have usually been made larger than 
paving-bricks, the common size being 12 inches long, 
3 or 4 inches w4de, and 4 or 5 inches deep. Thej^ are 
laid in the same manner as brick, as closely in contact 
as possible, and driven together. Under the action of 
the sun and the traffic, the asphalt blocks soon become 
cemented together through the medium of the asphaltic 
cement, and form, like the sheet asphalt pavements, a 
practically impervious surface. They are often laid 
upon gravel base, although in the best work a light 
concrete foundation is employed. 

In forming the asphalt block pavement the road-bed 
is brought to subgrade in the ordinary manner and 
rolled, leaving room for the pavement of uniform thick- 
ness to be placed upon it. A layer of gravel 4 or 5 
inches deep is then placed and rolled, or a base of con- 
crete is formed, with a cushion coat of sand I to 2 



29« A TEXT-BOOK ON ROADS AND PAVEMENTS. 

inches, and then the paving blocks. The blocks are 
pressed together in the courses by the use of a lever, 
and the courses driven against each other with a maul 
to reduce the joints as much as possible. A coating 
of sand is given to the surface of the pavement, and it 
is rammed to a firm and uniform surface, as in the case 
of brick. 

These blocks have the advantage over sheet asphalt 
for the smaller cities, that the blocks may be formed 
at a central point and shipped ready for use to the site 
of the proposed pavement, and that no special plant 
need be erected in each town where they are to be 
constructed. They have given satisfaction in use, and 
have frequently shown good durability in wear under 
moderate traffic. It is claimed that they are less 
slippery and may be used upon steeper slopes than 
sheet asphalt. The cost of transportation of the 
blocks makes this pavement expensive in manj^ locali- 
ties not in close proximity to the place of manufacture, 
and prevents them from competing successfully with 
other pavements. 

Art. 72. Maintenance of Asphalt Pavements. 

To give good service asphalt pavements must be 
kept clean. On account of the smooth surface and 
absence of joints, cleaning may be readil}' accomplished; 
and the presence of dirt, especially in wet weather 
when it is likely to cause the surface to remain damp, is 
liable to cause the asphalt to rot. More than any 
other pavement, therefore, the durability and wear of 
an asphalt surface depends upon its cleanliness. The 
presence of dirt upon asphalt in damp weather is also 
important in its effect upon the slipperiness of the 
pavement. 



ASPHALT PAVEMENTS. 299 

Small repairs of any breaks that may occur in an 
asphalt surface may be easily made, and such repairs 
should be constantly attended to in order to keep the 
surface in good condition. Small breaks will rapidly 
extend if they are not repaired at once. In making 
repairs to the surface of the pavement it is necessar3^ to 
cut away the surface for a short distance about the 
imperfect spot, stripping the surface from the founda- 
tion and cutting the layer down square at the edges, 
after which a new piece of surface ma3^ be introduced 
to fill the hole in the same manner that the original 
surface was constructed. Such a patch ma}^ ordinarily 
be put on so as to make joints that will join perfectly 
with the old pavement and not show where it has been 
placed. When a surface has become so worn that 
patches would be numerous, the old surface may be 
stripped off and a new one placed upon the original 
foundation. When repairs are to be made upon a 
pavement having a bituminous base it is more difficult 
to cut out the holes m satisfactory shape, as there is 
no well defined joint between the base and the surface 
layers. 

The repairs that may be required upon an asphalt 
pavement depend, of couise, upon the solidity of con- 
struction and the nature of the surface material. There 
is so great variation in the materials emplo^^ed for the 
wearing surface that, as would naturally be expected, 
very considerable difference in wear is shown by dif- 
ferent pavements. 

It is common to require contractors for asphalt 
pavements to guarantee the pavement for a period of 
3^ears, making all necessary repairs and leaving the 
work in good condition at the end of the period. This 
makes it an object for the contractor to do good work. 



300 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

and may sometimes be the most effective way of secur- 
ing it where so many elements of uncertainty enter. 
In general, it is not desirable to require contractors to 
guarantee paving for a long period on account of 
limiting competition and increasing unnecessarily the 
cost of the work. With asphalt paving, however, 
many engineers consider the difficulty of control during 
construction, under ordinary circumstances, such as to. 
make a guaranty necessary, while the fact that the 
material is for the most part controlled by a few large 
companies renders the guaranty less undesirable as 
restricting competition. This method has, however, 
been found unsatisfactory in many instances, on account 
of the difficulty of enforcing the guaranty. 

The cost of maintenance of asphalt pavements varies 
widely in different places, depending upon the character 
of the construction used and the local conditions sur- 
rounding the pavement. In Washington, D. C, the 
average life of the surface before renewal is about 20 
years, w^hile the annual cost of maintenance is about 
2.5 to 2.8 cents per square yard per annum. In loca- 
tions where the surface is kept continuously damp, 
particularly if it is not kept clean, the asphalt is apt to 
deteriorate rapidly and, in some instances, scales off 
and gradually disintegrates. The resistance of asphalt 
to water action depends very much upon the density 
of the surface mixture and the ease with which water 
may penetrate it. Great care should be used in laying 
pavements where moisture conditions are not good to 
secure a dense surface mixture in which the voids are 
well filled. Where water ma}^ continuously run in the 
gutters, it is usually better to construct the gutters of 
other material less affected by the action of water. 

Injury to asphalt surfaces from- illuminating gas 



ASPHALT PAVEMENTS. 301 

escaping from leaking mains has been observed by Mr. 
A. W. Dow at Washington, D. C. The heavy hydro- 
carbons of the gas are absorbed by the bitumen of 
asphalt, which is thereby softened and caused to cut 
and flow under the traffic. 

The cost of maintenance depends largely upon the 
system emplo^^ed in the maintenance work. In some 
cities repairs are made only at considerable intervals 
when the surface is in bad condition, and in such 
instances the ultimate cost is usually much larger than 
where small repairs are made as they are needed to keep 
the surface always in good condition. 

Art. 73. BiTULiTHic Pavement. 

The name '' bitulithic " is commonly applied to a 
pavement, the surface of which is composed of a 
bituminous concrete, the aggregate being a mixture of 
several sizes of broken stone, so proportioned as to 
give a dense material with a small percentage of voids. 
Pavements of bituminous concrete have been occasion- 
alh^ constructed for a number of years, but the intro- 
duction of this type of pavement upon a considerable 
scale began about 1901, when exploited under a patent 
of the Warren Brothers, and most of those since con- 
structed have been under this patent. 

In the construction of pavements of this class the 
crushed rock is screened into several sizes, which are 
then mixed together in such proportions as to produce 
an aggregate with very small percentage of voids. 
Four to six screens are used, varj^ing from about I J 
inches to yV inch openings. Sufficient quantities of the 
smaller sizes are employed to fill the interstices in the 
larger sizes; the relative proportions being determined 



302 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

in each instance by experiment upon the particular 
material in use. "After the proportions have been 
determined, the mineral material is passed through a 
rotary screen which separates it into several different 
groups of sizes. The proper proportion by weight of 
each of these sizes is secured by the use of a scale having 
seven beams, the exact required amount being weighed 
out and run into a double shaft rotary mixer. There 
it is combined with a bituminous cement which is also 
accurately weighed in the proper proportion. The 
whole is then thoroughly mixed together and dumped, 
while still hot, into carts, hauled to the street, spread, 
and thoroughly rolled with heavy steam road rollers. 

"After the surface is thoroughly rolled, a flush coat 
of quick drying bituminous cement is applied to the 
surface. There is then applied a thin layer of hot 
finely crushed stone, varying from i to } inches in 
size, according to the roughness of the surface desired. 
The pavement is again heavily rolled, leaving the street 
in a finished condition. " 

These pavements are commonly constructed upon 
bituminous foundations (see Art. 58). When the sub- 
foundation is not firm, and concrete foundations are 
required, the surface of the concrete is roughened by 
scattering stone of about 1 1 inch diameter lightly over 
it, and ramming the stones into the concrete to about 
half their depths. This forms a bond between the base 
and surface of the pavement, and prevents the creeping 
of the surface. 

These pavements have been used with good success 
in many places throughout the United States. They 
require care and skill in construction, both in securing 
proper grading of the mineral aggregate and in the 
character and proportions of the bituminous cement. 



ASPHALT PAVEMENTS. 3^3 

It is claimed b^^ the advocates of this kind of construc- 
tion that, on account of the densit}^ and firmness of the 
mass of stone of which they are composed, a softer 
bitumen maj^ be employed, thus eliminating the danger 
of cracking in cold weather. In some instances, where 
hard stone has been used in forming the surfaces, there 
are indications that this construction will give better 
resistance to wear than the ordinary asphalt surface, 
but longer experience is necessary to fulty test its 
durability. It has been successfully used upon much 
steeper grades than sheet asphalt, being reported as 
affording a good foothold to horses, and satisfactory 
in one instance upon a 12 per cent grade. 

In constructing these pavements, as with sheet 
asphalt, it has been customar}^ to rely upon the con- 
tractor's guaranty for securing good work and no 
attempt is usually made to determine the character of 
the bituminous cement b}^ direct tests. This is an 
undesirable feature of most work with these materials, 
and it is to be hoped that, as better information con- 
cerning the bitumens becomes available, more satis- 
factorj" specifications may become feasible. The fol- 
lowing is an extract from the specifications used in 
St. Louis in 1908: 

''Upon the foundation shall be laid the wearing 
surface, which shall be composed of carefully selected 
sound, hard crushed stone, mixed with bituminous 
cement and laid, as hereinafter specified. 

" The stone last referred to shall have a percentage of 
wear not to exceed 5 per cent when tested in the follow- 
ing manner: 

"The sample to be tested shall be broken into pieces 
that will pass, in all positions, through a 6 centi- 
meter ring, but not through a 3 centimeter ring. The 



304 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

fragments of stone shall then be cleaned, dried in a hot 
air bath at 100° C. and cooled in a dessicator, after 
which five kilograms shall be weighed out and placed 
in a cylinder of an abrasive machine and the cover 
bolted on. This machine (see Art. 37) shall consist 
of a cast iron cylinder, or cylinders, fastened to a shaft 
so that the axis of each cylinder makes an angle of 
30 degrees with the axis of rotation. Each cylmder 
shall be 20 centimeters in diameter and 34 centimeters 
in depth; shall be closed at one end and shall have a 
tightly fitting cover at the other end. After this, 
the machine shall be rotated at the rate of 2000 revo- 
lutions per hour for five hours. When the 10,000 
revolutions of the machine are completed, the contents 
of the cylinder shall be placed on a sieve of 0.16 centi- 
meter mesh, and the material which passes through 
carefully collected and weighed. The ratio between 
the weight of the fine material and the original five 
kilograms placed in the cylinder is the percentage of 
wear. 

" After immersion in water for a period of ninety-six 
hours, a smoothly worn fragment of stone w^eighing 
between 20 and 60 pounds shall not absorb more than 
3 pounds of water per cubic foot of stone. 

"After heating the stone in a rotary mechanical 
dryer to a temperature of about 250° F. it shall be 
passed through a rotary screen having six or more 
sections, with varying sized openings, the maxi- 
m.um of which shall not be larger than one and 
one-half inch, and the minimum one-tenth of an inch 
in diameter. The several sizes of stone thus separated 
1)y the screen sections shall pass into a bin containing 
six sections or compartments. From this bin the 
stone shall be drawn into a weight box, resting on a 



ASPHALT PAVEMENTS. S^S 

scale having seven beams. The stone from each bin 
shall be accurately weighed in the proportions deter- 
mined by laboratory tests that will give the greatest 
density of mineral aggregate and the greatest inherent 
stability of the mineral aggregate. From the weigh- 
box each batch of mineral aggregate composed of 
different sizes, accurately weighed, as above described, 
shall pass into a "twin pug" or other appropriate 
form of mixer. If the proportions of crushed stone in 
the mixer do not provide enough fine particles to bring 
the aggregate to the density desired, there may be 
added not to exceed 15 per cent of fine sand, gravel, 
hydraulic cement, and pulverized limestone. To the 
stone in the mixer shall then be added a sufficient 
quantity' of Warren's Puritan Brand No. 21 Bituminous 
Water-proof Cement, to thoroughly coat all the particles 
of stone and fill all the voids in the mixture. The 
bituminous cement shall, before mixing with the stone, 
be heated to between 200 and 250° F. and the amount 
used in each batch shall be accurately weighed and 
used in such proportion as have been previously deter- 
mined by laboratory tests to give the best results 
and fill the voids in the mineral aggregate. The 
mixing shall be continued until the result is a uniform 
bituminous concrete. In this condition it shall be 
hauled to the street and there spread on the prepared 
foundation to such depth that after thorough com- 
pression with a steam roller it shall have a thickness 
of two inches. The proportion of the various sizes of 
stone and of bituminous cement shall be such that the 
compressed mixture shall have as nearly as possible 
the density of solid stone. 

'"After rolling the wearing surface, there shall be 
spread over it a thin coating of Warren's Quick 



3o6 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Drying Bituminous Flush Coat Composition in a plas- 
tic condition, for the purpose of closing any pores or 
cellular openings, and to thoroughly fill any uneven- 
ness or honeycomb which may appear in the surface. 
There shall then be applied thereto and combined 
therewith while plastic, stone chips, with the same 
qualities required of the stone in the pavement 
proper, by rolling the same into the surface with 
a heavy steam roller for the purpose of presenting a 
gritty surface. 

"In order to get the greatest possible density, the 
pavement shall be rolled continuously from the time the 
bituminous concrete is brought upon the street until 
the stone chips have been rolled into the surface and 
the roller no longer makes a perceptible impression 
upon the pavement. 

" Each layer of the work shall be kept as clean as 
possible so as to readily unite with the succeeding 
layer. The bituminous compositions shall in each 
case be free from water, petroleum oil, water gas, or 
process tars and shall be especially refined with a view 
of removing the light oil, naphthaline and other crys- 
talline matter susceptible to atmospheric influences/* 



CHAPTER X. 

WOOD-BLOCK PAVEMENTS. 
Art. 74. Types of Wood-Block Pavement. 

The use of wood blocks for the surfaces of pave- 
ments began a little before 1840, and since that time 
many types of construction have been tried with vary- 
ing degrees of success. The first pavements in London, 
in 1839, consisted of hexagonal blocks of fir, six to 
eight inches in diameter and about six inches deep, 
placed on a base of gravel. In 1 841 a pavement of 
round beech blocks was laid upon a foundation of 
planks and sand. The wood soon decayed and the 
pavement was removed. 

In Philadelphia, square hemlock blocks were laid in 
1839 and hexagonal hemlock blocks probably a little 
earlier. Both wxre quickly destroyed by the decay of 
the blocks. In New York and Boston similar pave- 
ments were constructed at about the same time and 
with much the same result. 

In 1855 a pavement of tamarac blocks was laid in 
Quebec, This pavement was placed upon a base 
formed of a flooring of one and one-half inch boards 
laid longitudinally and crossed at right angles by a 
second flooring of inch boards. A layer of sand one- 
half inch thick w^as placed over the boards. The 
tamarac blocks were ten to fifteen inches in diameter 
and twelve inches long, small pieces of wood being 
forced into the spaces between the blocks. The 

307 



3o8 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

joints were filled with a mixture of sand, cement, and 
tar. This heavy construction is reported as having 
given very good wear, with no decay. 

Cedar Block Pavements. In the earlier wood pave- 
ments of the United States, cedar blocks were com- 
monly employed. These blocks were used in the form 
of whole sections of the tree on account of the liability 
of the wood to split off between the layers when cut to 
a rectangular shape, as well as to reduce waste to a 
minimum. They usually varied from 4 to 9 inches in 
diameter and 4 to 8 inches in depth. In some cases 
the blocks were cut to a true cylindrical form, the sap- 
wood as well as the bark being cut away by passing 
the block through sets of knives, gauged to turn out 
true cylinders of given size. The use of sapless blocks 
increases the life of the pavement by augmenting the 
resistance of the material both to the wear of traffic and 
to the disintegrating influences of the atmosphere. 

These pavements were usually placed upon a founda- 
tion of boards laid upon sand. The planks were com- 
monly tarred and laid lengthwise of the street, being 
nailed to scantling or other boards placed across the 
street and bedded in the sand. This construction has 
the disadvantage of lacking firmness as well as of being 
perishable, although in some instances good results have 
been obtained by its use. 

The construction of a pavement of this type is shown 
in Fig. 25. Blocks of varying sizes are employed, 
being set in contact with each other in such a way as 
to leave the spaces between the blocks as small as 
possible. Usually the joints are filled with sand and 
gravel, sometimes with a coating of tar; or in some 
cases the joint is partially filled with tar and then com- 
pletely filled with sand or small gravel. When the 



WOOD-BLOCK PAVEMENTS. 



309 



ordinary coal-tar paving cement filling is used, the 
joints are first filled nearly full of sand or gravel, which 
is pounded down with a bar, after which the hot cement 
is poured in until the joint is well filled. 

These pavements, on account of the plentiful supply 
of timber were constructed for very low first cost and 
undoubtedly served a very useful purpose in many 
instances, permitting the improvement of many streets 
to an extent which at that time would have otherwise 
been impossible. They lifted the streets out of the 
mud, although the pavements did not usually last long 




Fig. 25. 

and were afterward replaced by more durable materials^ 
Pavements of this type wear rapidly under traffic, soon 
becoming uneven, and their use has, for the most part, 
been discontinued on account of their lack of economy. 
Nicholson Pavement. Rectangular wooden blocks 
set like the cedar blocks, upon a plank foundation were 
at one time quite extensively used and known as 
Nicholson pavements. In these pavements the blocks 
are set with their longest dimension transverse to the 
length of the street. They are usually arranged in 
courses across the street, being placed close together 
in the courses, and arranged to break joints in adjoining 
courses. Between courses a ioint is usually made \ to 



3IO A TEXT-BOOK ON ROADS AND PAVEMENTS. 

i inch in width for the purpose of affording a foothold 
to horses. In the older pavements of this character a 
much wider joint was employed, some as much as an 
inch in width, with the idea that they were necessary 
to secure proper foothold. The joints were filled in the 
same manner as in the round block pavement. These 
pavements like the cedar blocks have given place for 
the most part to more economical kinds of construction. 
Rectangular Blocks on Concrete. The use of a concrete 
base under a surface of the Nicholson type effected a 
marked improvement in the wear of the pavement. 




P"IG. 26. 

Round block pavements were also sometimes placed 
upon a concrete foundation. 

In using a concrete foundation a cushion coat of 
sand is commonly employed on top of the concrete in 
which to bed the blocks in order that they may be 
brought to an even surface. Sometimes a thin la^^er 
of cement mortar is used in place of the sand upon the 
concrete; and in London some pavements have been 
constructed with a thin layer, about \ inch, of asphalt 
mastic over the concrete, the blocks resting upon thj 
mastic. 

A pavement of this type is shown in Fig. 26. 

In laying a pavement of this kind a course of blocks 



WOOD-BLOCK PAVEMENTS. 311 

is first set across the street, and then a strip of wood 
of the thickness of the joint is set against the row of 
blocks and left until the next course is placed, or 
sometimes spuds with heads of the thickness of the 
joints are driven to the head in the side of each block, 
and the next row of blocks are set against the spuds. 

In some pavements of this kind hydraulic cement 
is employed in filling the joints, and in some instances 
the lower half of the joint is filled with coal tar paving 
cement and the upper half with hj^draulic cement 
mortar. The cement mortar gives a harder wearing 
surface, and protects the pitch from the softening action 
of the sun in warm weather. In later practice the 
width of joint has been graduall3^ reduced until the 
blocks are set in contact with each other, occasional 
expansion joints being provided. 

These pavements have been extensively used in 
England, and to a smaller extent in the United States. 
The3^ have been fairl^^ satisfactory in use but have 
been, for the most part, superseded by treated blocks. 

Treated Block Pavement. Wood blocks treated by 
some preservative process for the purpose of preventing 
decay and of hardening the block so as to give better 
resistance to wear have come into use somewhat 
extensively since about 1900. These pavements have 
given good service in use, under heavy traffic in the 
business sections of the large cities, and on important 
residence streets, where the somewhat high cost is not 
prohibitive. The use of untreated blocks has been prac- 
tically abandoned as uneconomical. 

Art. 75. Wood Blocks. 

Wood-block pavements are constructed of blocks set 
with the fibers vertical, so that wear comes upon the 



312 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

ends of the fibers and has no tendency to spHt pieces 
off from the blocks. These blocks are usually from 6 
to 12 inches in length, 3 to 4 inches in width, and 3 to 4 
inches in depth. Some engineers require the blocks to 
be of uniform length but a variation of from about 6 to 
10 inches is more common and seems desirable because 
of the greater freedom in obtaining timber for the pur- 
pose. A depth of 3 J inches, or at most 4 inches, is 
sufficient and there seems to be no advantage in greater 
depth, as the block would become unserviceable and 
need to be renewed before this depth would be worn 
away. The blocks are cut from planks of uniform 
thickness, and are set in courses across the street, the 
blocks in adjoining courses breaking joints with each 
other. 

Kinds of Wood. Wood for pavements should be 
close-grained and not too hard. It should be as homo- 
geneous as possible in order that the wear may be 
uniform, and soft enough that it may not wear smooth 
and slippery. The blocks should always be subjected 
to careful inspection, and only sound and well-seasoned 
timber should be employed. Blocks containing shakes 
and knots should be rejected, and when untreated blocks 
are to be used, all sapwood needs to be removed in order 
to lessen the liability to early decay. 

In Australia hard-wood blocks have been quite 
extensively used and are reported as giving good ser- 
vice, although they are admitted to be somewhat slippery 
in wet weather. Australian Karri and Jarrah woods 
are employed, and it is claimed for them that they show 
unusually great resistance to wear and are not soon 
affected by decay. These woods are too dense for pre- 
servative treatment and are used in the form of untreated 
blocks. 



WOOD-BLOCK PAVEMENTS. 313 

In London, where wood pavements have been very 
extensively employed, Swedish yellow deal is commonly 
placed at the head of the hst of woods in value, yellow 
pine and Baltic fir being also largely used and considered 
good in use. The Australian woods above mentioned 
have also been used to some extent in London, and are 
said to have given very satisfactory servdce, show- 
ing greater resistance to wear than deal or pine, although 
somewhat expensive. Deal treated with creosote is 
extensively used and seems to give the best satisfaction. 
In Paris, teak, karri, and pitch pines are frequently 
employed, although treated native pines are more 
commonly used and have been found to give good 
service. 

In the United States, Southern yellow pine has been 
most extensively employed, and seems to have given the 
best satisfaction in use, but is so much in demand for 
other purposes that properly selected timber is rather 
expensive. Norway pine, tamarack and Southern black 
gum are also used to some extent and have given good 
service. White birch and hemlock have also been 
successfully tried for this purpose, although they have 
only been used in experimental pavements. All of 
these woods are used as treated blocks, and are readily 
susceptible to impregnation with the creosote oils used 
in treatment. 

The specifications used in New York City in 191 1 
require the blocks to be of Southern long-leaf yellow pine 
or of Southern black gum, while the St. Louis specifica- 
tions permit the use only of the Southern loQg-leaf yellow 
pine blocks, which must meet the following requirements : 

'' Each block shall consist of at least seventy-five 
(75) per cent heart, and shall be free from bark, large, 
loose or rotten knots, and shakes or other defects. No 



314 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

dead or second-growth timber will be accepted, and all 
timber shall be cut in a locality recognized as producing 
the timber described. 

" The number of annual rings shall average not less 
than six (6) per inch, measured radially. All timber 
for the blocks shall be thoroughly air-seasoned." 

It is customary to require that the timber shall be 
subject to inspection at the works, before being cut 
into blocks, or treated with oil. 

The specifications adopted by the Association for 
Standardizing Paving Specifications in 191 1, contain 
the following requirements: 

" The wood to be treated shall consist of Southern 
yellow pine, Norway pine, black gum and tamarack, 
only one kind of wood, however, to be used on any one 
contract. 

'' Yellow pine blocks shall be made from what is 
known as Southern yellow pine, well manufactured, 
full size, saw-butted, all square edged, and shall be free 
from the following defects: unsound, loose aad hollow 
knots, worm holes and knot holes, through shakes and 
round shakes that show on the surface. In yellow pine 
timber the annual rings shall average not less than eight 
to the inch, and shall in no case be less than four to the 
inch measured radially. 

" Norway pine block, gum and tamarack block shall 
be cut from timber that is first class in every respect, and 
shall be of the same grade as that defined for the Southern 
yellow pine. 

" The blocks shall be from 5 to 10 inches long, but 
shall average 8 inches; they shall be three and one-half 
(3i) and four (4) inches in depth, according to traffic; 
they shall be from three (3) to four (4) inches in width; 
but all blocks in one street or improvement shall be of 



WOOD-BLOCK PAVEMENTS. 315 

uniform width, provided that blocks 3 inches in depth 
can be used on residential streets and in alleys; pro- 
vided, further, that in no case shall the width and depth 
of blocks be equal. 

" A variation of one sixteenth {^) of an inch shall be 
allowed in the depth and one eighth (J) of an inch in the 
width of the blocks." 



Art. 76. Treatment of Wood Blocks. 

The most serious objectioQ commonly raised to the 
older type of wood pavement is that wood, being porous, 
absorbs moisture readily, and is thus liable both to 
destruction through decay and to become injurious to 
health. Various methods were therefore proposed for 
rendering the blocks less pervious and more durable 
by impregnating them with various substances which 
fill the pores and act as preserv^atives. The earher 
attempts in this direction were not in the main success- 
ful and little seemed to be gained in durability by the 
treatment. Solutions of mineral salts were tried but 
were found unsuitable for the purpose. Creosoting the 
blocks, which consists in impregnating the wood with 
the oil of tar, or creosote, was more successful, but 
with the type of construction in use seemed of doubt- 
ful economic value. 

In the process of creosoting, the wood is first thor- 
oughly dried, usually by heating it in a kiln, and the 
hot creosote is then forced in under pressure. The 
method of accomplishing this varies in different places. 
In order to be effective the process must be thoroughly 
carried out and the pores well filled. It has been com- 
monly recommended that from 8 to 12 pounds of creosote 



3i6 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

per cubic foot of timber should be forced in, as a min- 
imum requirement for the softer woods, such as are 
commonly used in pavements. Creosote has the prop- 
erty of destroying the lower forms of animal hfe, and is 
therefore an effective preservative against destruction 
through these agencies where they exist. It is therefore 
often employed for the preservation of timber for sub- 
aqueous construction in sea-water. This process, when 
properly applied, is effective in preventing decay, and 
therefore in lengthening the natural life of the wood. 
It also renders the wood less perraeable and thus removes 
the objection to the old form of pavement based upon 
its absobent naUire. 

The earher efforts to increase the life of wood pavements 
by the use of creosoted blocks were rather unsuccessful. 
The resistance of the wood to the wear of the traffic did 
not seem to be materially affected by the treatment given, 
and the failure of the pavement being ordinarily from 
wear rather than from decay, the Hfe of the pavement 
was not materially prolonged. More recently, however, 
careful methods of treatment have produced blocks 
which show high resistance to wear, and give good results 
under the heavy traffic of the large cities. The present 
treatment is more thorough, a larger quantity of oil 
being injected, and in most instances, heavier oils being 
employed. The object is to waterproof the blocks by 
completely filling the pores of the wood. It is also 
claimed that heavy oils harden in the pores and add to 
the resistance to wear by preventing displacement of the 
wood fiber, when subjected to loads or bloAvs. 

The heavy oil of tar, known as creosote oil in the 
preservation of timber for other purposes, is commonly 
used, but is frequently combined with other materials 
intended to give greater stability and more thorough 



WOOD-BLOCK PAVEMENTS. 317 

waterproofing. There is much difference of opinion 
concerning the efficiency of the various oils employed. 

Creo-resinaie Process. Many of the pavements now 
in use have been constructed by the creo-resinate process. 
In the creo-resinate treatment, the wood is impregnated 
with a substance consisting essentially of a mixture of 
the oil of tar with resin, the resin acting as the water- 
proofing and hardening material. The amount of resin 
required varies from about 25 per cent to 50 per cent 
of the mixture, and depends upon the character of the 
oil used, a heavy dense oil requiring less resin than a 
lighter and more volatile oil. The method of treat- 
ment is thus described by Mr. F. A. Kummer: * 

" Blocks, after being cut to size, are placed in circular 
cages made of band steel of approximately the diameter 
of the cylinders in which the treatment takes place, 
and are then, while in these cages, run into the cylin- 
ders on cars. The cylinders themselves are usually 
about 6 feet in diameter and somewhat over 100 feet 
long, and are provided with steam coils along the bot- 
tom and sides to provide heat for drying and preparing 
the lumber for treatment. The blocks are heated in 
this way, some works employing live steam instead of 
steam coils, and others a combination of the two. After 
several hours both by the use of heat and by the use of a 
vacuum pump, a large portion of the moisture and light 
volatile oils in the wood, if the latter contain any such, 
are driven oft*. The preservative material is run into 
the cylinder under a vacuum and hydrauKc pressure 
of 200 pounds per square inch, applied from two to three 
hours, or for such longer period of time as may be nec- 
essary to thoroughly treat the charge, the result being 

* Engineering Record, August 25, 1906. 



3I« A TEXT-BOOK ON ROADS AND PAVEMENTS. 

accomplished when the gauges show that no more material 
is entering the wood." 

This method was specified for a number of years in 
the City of New York, but is no longer required on 
account of the high cost. 

Creosote Oil. Sometimes so-called creosote oil is used 
without the admixture of heavier materials. The advocates 
of this method claim that a more thorough impregna- 
tion of the wood is possible than with the heavier materials, 
that the results are equally good in durability, and that 
the material is m.ore easily obtainable. The character 
of the oil used is shown by the following extract from 
specifications recommended by the Wyckoff Pipe and 
Creosoting Company. These specifications require the 
injection of at least i8 pounds of the oil into yellow pine 
blocks : 

" The creosote oil shall be a dead oil of coal tar or 
coal-tar product. It shall not contain more than 3 per 
cent of water and if it does contain this amount of water 
a corresponding correction must be made so that an 
equivalent additional amount of creosote is forced into 
the blocks. It shall contain only traces of acetic acid and 
acetates. Its specific gravity at 100° F. (38° C.) shall 
be at least 1.03 and not more than 1.07 so as to assure 
its thoroughly penetrating the wood blocks. The residue 
insoluble by filtration with benzol and chloroform must 
not exceed 3 per cent of the weight of the creosote oil. 
Fractional distillation of 100 grams of the creosote oil 
shall produce percentages of dry oil by weight within the 
following Hmits: 

Up to 150° C. (302° F.) not to exceed 2% 

Between 150° C. (302° F.) and 170° C. (338° F.) not to exceed 1.5% 
Between 170° C. (338° F.) and 235° C. (455° F.) not to exceed 35% 
Between 235° C. (455° F.) ar.d 300° C. (572° F.) not to exceed 35% 



WOOD-BLOCK PAVEMENTS. 3 19 

The residue remaining shall be soft and adhesive. 
The creosote oil shall contain about 25 per cent of 
crystalHzable naphthalene and at least 15 per cent anthra- 
cene oils. At least 95 per cent of the creosote oil shall 
be soluble in carbon bisulphide and equally in absolute 
alcohol." 

Heavy Tar Oils. The use of tar oils of greater specific 
gravity than the ordinary creosote oils, has been rapidly 
developing in the past few years. These c Is are pro- 
duced by adding coal-tar pitch to creosote oil so as to 
obtain an oil of proper consistency. It is claimed by 
advocates of these oils that they more completely water- 
proof the block, and make it more resistant to wear. 
The Association for Standardizing Paving Specifications 
has recommended this method of treatment. These 
specifications have been criticised on the ground that the 
oil is difficult to obtain and is a monopoly. This, how- 
ever, the association does not think to be the case. The 
objection is also made that these heavy tar oils make a 
pavement which becomes sticky and disagreeable in 
hot weather. The association, however, are of opinion 
hat this is the most desirable method of treatment, and 
in 19 1 1 adopted the following specifications: 

Preservative. The preservative to be used shall be a 
coal-tar product, free from adulteration of any kind 
whatever, and shall comply with the following require- 
ments : 

1. The specific gravity shall not be less than i.io or 
more than 1.14 at a temperature of thirty-eight (38) 
degrees Centigrade. 

2. Not more than three and one-half (3^) per cent of 
the oil shall be insoluble by hot continuous extractions 
with benzol and chloroform. 

3. On distillation, which shall be made exactly as 



320 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

described in Bulletin No. 65 of the American Railway 
Engineering aad Maintenance of Way Association, as 
shown in the appendix to these specifications, the dis- 
tillate shall not exceed two (2) per cent up to 150 degrees 
Centigrade, and shall not be less than thirty (30) or 
more than forty (40) per cent up to 315 degrees Centigrade. 

4. The manufacturer of the oil shall permit full and 
complete inspection and samphng at the factory at which 
the oil is produced, of all materials either crude or refined, 
entering into the manufacture of the finished product, 
as well as the finished product itself, in order that the 
materials used can be determined to be in accordance 
with the foregoing requirements. He shall also submit 
satisfactory proof of the origin of all materials entering 
into the composition of the finished product. 

Samples of the preservative taken by the inspector from 
the treating tank during the progress of the work shall 
at no time be allowed to show an accumulation of more 
than two (2) per cent of foreign matter, such as sawdust 
and dirt. 

Due allowance shall be made for such accumulation 
of foreign matter by injecting an additional quantity 
of oil into the blocks. 

Treatment. The blocks shall be treated with the 
preservative elsewhere described, so that the pine and 
tamarack blocks shall coatain not less than twenty (20) 
pounds, and the gum blocks not less than twenty- two 
(22) pounds per cubic foot. 

This amount may be reduced to sixteen (16) pounds, 
under conditions of heavy traffic in the discretion of the 
engineer. 

Inspection. The party manufacturing the blocks shall 
equip his plant with all necessary gauges, appliances 
and facihties to enable the inspector to satisfy himself 



WOOD-BLOCK PAVEMENTS. 321 

that the requirements of the specifications are ful- 
filled. 

Art. 77. Tests for Wood Blocks. 

Specification requirements for wood blocks vary 
widely in different places throughout the country and 
no systematic method has been adopted for the inspec- 
tion and testing of the blocks. A number of tests have 
been proposed for use both at the plant where the blocks 
are treated and after the blocks have been dehvered at 
the site of the pavement. Inspection of the blocks 
is frequently made before treatment, as well as tests 
of the oil to be used in the treatment, an inspector being 
kept at the plant for the purpose while the blocks are 
being prepared. The thoroughness of the treatment 
is determined by the difference in weight of treated 
and untreated blocks. 

For the examination of Avood blocks after their delivery 
at the point of use, several tests are in use or have been 
proposed : 

a. The blocks are inspected as to their size, shape, 
aad freedom from defects. 

b. Blocks may be split and examined as to the thor- 
oughness of the treatment, and a weight test applied 
to determine whether a sufficient quantity of oil has been 
absorbed by the block. 

6. Tests of absorption are made by first drying the 
blocks and then soaking them in water, thus determining 
the amount of water that may be absorbed. 

The 191 1 specificatioQS of the City of New York con- 
tain the following requirement: 

" After treatment the blocks are to show such water- 
proof qualities that after being dried in an oven at a 
temperature of 100° F. for a period of 24 hours, weighed 



322 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

and then immersed in clear water for a period of 24 hours 
and again weighed, the gain in weight is not to be more 
than 3i per cent for pine blocks and 4 J per cent for gum 
blocks." 

This test is made upon blocks as delivered for use in 
order that the effect of drying out after treatment may be 
included in the results. 

d. The character of the oil with which the block has 
been treated is tested by extracting the oil with carbon 
bisulphide and then subjecting it to tests to determine 
whether it conforms to the specifications for oil to be 
used in the treatment. Fine turnings from the block are 
placed in an extraction apparatus with the solvent, and 
the oil completely extracted. The separation of the 
creosote oil from the solution is effected by distillation, 
the solvent being first removed at a temperature of about 
120° C, and the creosote oil below about 370° C. The 
creosote oil thus separated is then subjected to tests 
to determine whether it meets the specification require- 
ments. 

e. It has been proposed to test the resistance to 
abrasion of the blocks by grinding them upon a disk 
machine, but no records are available as to results obtained 
in such tests, and they are of doubtful utility. 

A standard method of making analyses of creosote 
has been adopted by the American Railway Maintenance 
of Way Association, and is now commonly employed in 
making such examinations. 

Art. 780 Construction of Wood Pavements. 

As stated in Art. 74 the older types of wood-block 
pavement, in which the blocks were laid with open 
joints on a plank foundation or on gravel, are prac- 



WOOD-BLOCK PAVEMENTS. 323 

ticalh^ obsolete, and wood blocks are usually laid with 
close joints upon concrete foundations. This gives 
firm support to the blocks and admits of even wear 
upon the surface of the pavement. A durable base 
also has the advantage that when the surface layer is 
worn out, the pavement may be resurfaced without 
removing the foundation. The concrete base is con- 
structed in the ordinary manner as described in Art. 57. 
It is commonly about 6 inches thick, although under 
specially trying conditions a somewhat greater thick- 
ness is sometimes emplo3^ed. In a few of the European 
pavements very heavy foundations, 7 or 8 inches thick, 
are employed; but these are exceptional and the 6 inch 
depth is usually found sufficient. Lighter foundations, 
4 or 5 inches in depth may be used under favorable 
conditions and where traffic is not heavy; but these 
pavements are usually employed upon streets of con- 
siderable traffic, and in such situations very light 
construction is not desirable. 

For the purpose of receiving the blocks and affording 
them uniform support, a cushion coat of sand or a 
thin coating of cement mortar is placed over the con- 
crete. The sand cushion when used is usually about 
I inch in thickness and is placed in the same marmer 
as in laying a brick pavement. When a mortar surface 
is employed, a coating of about i inch of mortar is 
floated over the surface of the concrete and brought to 
the exact form of the finished surface, the blocks being 
placed before the mortar sets and bedded into the 
surface of the mortar. This method, while used to a 
much less extent than the sand cushion, seems to give 
excellent results in maintaining a uniform surface 
where the work is properly done, giving more uniform 
support than the sand cushion. 



324 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

The blocks are set with the grain vertical, close 
together and commonly in courses making an angle 
of 60 to 70 degrees with the curb line. In some 
instances the blocks are placed with open joints across 
the street of J to | inch. Most of the older work was 
constructed in this manner, the wide joints being 
intended to give better foothold to horses, as well as to 
allow for expansion. Expansion is commonly provided 
for by an expansion joint along the curb, although in 
some instances such joint is not used. These joints are 
filled with bituminous cement, and are usually about 
f of an inch wide for streets not more than 50 feet in 
width. Sometimes expansion joints are used across the 
street at distances of about 100 feet apart, although the 
use of such joints is diminishing, and under ordinary 
circumstances do not seem to be necessary. 

After the blocks are set, they are rolled with a small 
steam roller until the surface is smooth and even. When 
the blocks are placed on a mortar bed, the rolling must 
be completed before the mortar has set. 

Filling the Joints. Three methods of filling joints 
ate frequently employed, sand, pitch and cement mortar 
being used. Sand filler is applied by placing a light 
coating of dry sand over the surface and brushing it 
into the joints, then covering the pavement with a layer 
of sand and opening the street to traffic. Pitch joints 
are made by the use of asphalt or coal-tar paving cement 
as used for brick pavements. They are made by spreading 
the hot paving cement over the surface and brushing 
into the joints. Care must be used to apply the cement 
at such temperature as will cause it to readily run into 
the joints and brush off all surplus cement. A light 
coating of sand is then placed over the pavement to 
take up and grind off the pitch left on the surface of 



WOOD-BLOCK PAVEMENTS. 325 

the pavement, which may othenvise become objection- 
able in warm weather. 

When the joints are grouted with hydraulic cement, 
a mortar composed of one part cement to two parts 
sand is usually employed, mixed to a liquid condition 
so that it may easily run into the joints. This mortar 
is slushed upon the surface and broomed into the 
joints, and a light coating of sand is placed over the 
surface before opening to traffic. This sand is ground 
by the traffic into the blocks, tending to make the sur- 
face more gritty. 

Following are extracts from specifications recommended 
by a Committee of the American Society of Municipal 
Improvements in 19 10: 

^^ Laying Blocks. Upon the bed thus prepared the 
blocks shall be carefully set with the fiber of the wood 
vertical in straight parallel courses at right angles to the 
curb, except that one row of block shall be placed parallel 
with the curb and | of an inch therefrom. The space 
thus formed between the curb and this row of blocks 
shall be filled with a bituminous filler having a penetra- 
tion between 30 and 40 when tested at 77° F. On streets 
50 feet or more in width, a second row of blocks parallel 
to the first row along the curb and f of an inch therefrom, 
shall be laid and the space between the two row^s filled 
with a bituminous filler as above, thus forming a double 
expansion joint. 

'^ When deemed advisable by the engineer on streets 
for heavy traffic the row or rows of blocks parallel with 
the curb and the expansion joint may be dispensed with. 

'' The blocks should be laid by setting them loosely 
together on the cushion coat, but no joint shall be more 
than I of an inch in width. Nothing but whole blocks 
shall be used, except in starting a course or in such other 



326 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

cases as the city may direct, and in no case shall less 
than one-third of a block be used in breaking joints. 
Closures shall be carefully cut and trimmed by experienced 
men. The portions of the blocks used for closure must 
be free from check or other fracture, and the cut end 
must have surface perpendicular to the top of the block 
and cut to the proper angle to give a close, tight joint. 

^'After the blocks are placed, they shall be rolled by a 
small steam roller until the surface becomes smooth 
and is brought truly to the grade and contour of the 
finished pavement. When laid on a mortar bed, the 
rolling shall be completed before the mortar has set, 
and all mortar that has set before the blocks are in place 
and rolled, shall be discarded and replaced by fresh 
mortar. 

''After the blocks have been thoroughly rolled, the 
joints between them shall be filled by sweeping with fine, 
clean, dry sand, all of which shall pass a ten-mesh sieve. 

''After inspection by the proper city official, the surface 
of the wood block pavement shall be covered to a depth 
of about i inch with fine screened sand. This sand is 
to be left upon the pavement for such time as may be 
directed by the proper city official, after which it shall 
be swept up and taken away by the contractor." 

Art. 79. Maintenance of Wood Pavements. 

The ordinary maintenance of wood pavements, like 
that of most other pavements, consists in keeping the 
pavement clean and in repairing from time to time any 
small breaks that may appear in the surface due to im- 
perfect material or to the settling of the foundation. 
These repairs would, of course, include the removal of 
any defective blocks and the taking up and replacing of 



WOOD-BLOCK PAVEMENTS. 327 

any portion which may settle out of surface through 
inefficient support. 

It is generally agreed that the wear of a wood surface 
is improved by giving it an occasional coating of small 
gravel, in some cases two or three times a year, and 
permitting it to be ground into the surface for a few 
days. 

When the wood pavement needs renewal or exten- 
sive repairs the surface ma3^ be relaid as with any other 
block pavement: if a permanent foundation be em- 
ployed, by stripping the blocks from the foundation and 
placing a new surface in the same manner as the first 
one; with a board foundation that also must be relaid. 
Observations made by Mr. Kummer" seem to indi- 
cate that the continual wetting of a wood block sur- 
face tends to materially reduce the resistance to wear. 
Blocks from a street which had been sprinkled " instead 
of being pounded dow^n and dense and hard, as is the 
case on streets not so sprinkled, had broomed out under 
the action of travel and the preservative material 
mechanically pounded out of the wood by the com- 
bined action of the travel and Avater. This, of course, 
leaves the surface of the block unprotected by the 
antiseptic preservative and subject to decay. It also, 
in its spongy condition, offers poor resistance to wear. " 
The surface of wood block pavement. does not give off 
fine dust, and need only be sprinkled sufficiently to be 
swept clean. For the best results it is necessarj^ that 
the surface should be kept free from dirt. 

* Engineering Record, August 25, 1906. 



CHAPTER XI. 

STONE-BLOCK PAVEMENTS. 
Art. 8o. Stone for Pavements. 

Stone-block pavements are commonly employed 
where the traffic is heavy and a material needed which 
will resist well under wear. 

Stone for this purpose must possess sufficient hard- 
ness to resist the abrasive action of wheels. It must 
be tough, in order that it may not be broken by shocks. 
It should be impervious to moisture and capable of 
resisting the destructive agencies of the atmosphere 
and of weather changes. 

Experience only can determine the availability of 
any particular stone for this use. The stone may be 
tested in the same manner as brick, and perhaps some- 
thing predicated as to the probability of its wearing 
well under traffic; but the conditions of the use of the 
material in the pavement are quite different from those 
under which it may be tested, and any tests looking to 
a determination of its weathering properties are apt to 
be misleading. 

Examination of a stone as to its structure, the close- 
ness of grain, homogeneity, etc., may assist in forming 
an idea of its nature and value for wear. Observations 
of any surfaces which may have been exposed for a 
considerable time to the weather, either in structures 
or in the quarry, will be the most efficient method of 
forming an opinion concerning the weathering proper- 

328 



STONE-BLOCK PAVEMENTS. 329 

ties of the stone. The conditions of use in pavements 
are, however, somewhat different from ordinary expo- 
sure in structures, on account of the material in the 
pavement being subject to the action of water contain- 
ing acids and organic substances due to excretal and 
refuse matter. A low degree of permeability usually 
indicates that a material will not be greatly affected by 
these influences and also that the effect of frost will 
not be great. 

Granite and sandstones are commonly employed for 
paving blocks and furnish the best material. Lime- 
stones are sometimes used, but have seldom been found 
satisfactory. Trap-rock and the harder granites, while 
answering well the requirements as to durability and 
resistance to wear, are objectionable on account of 
their tendency to Wear smooth and become slippery 
and dangerous to horses. Granite or syenite of a tough, 
homogeneous nature is probably the best material for 
the construction of a durable pavement for heavy 
traffic. Granites of a quartzy nature are usually brittle 
and do not resist well under the blows of horses' feet or 
the impact of vehicles on a rough surface. Those con- 
taining a high percentage of feldspar are likely to be 
affected by atmospheric agencies, w^hile those in which 
mica predominates wear rapidly on account of their 
laminated structure. 

Sandstones of a close-grained, compact nature often 
give very satisfactor3^ results under heavy w^ear. They 
are less hard than granite and wear more rapidl^^ but 
do not become so smooth and slippery, and commonly 
form a pavement that is more satisfactory from the 
point of view of the user. Sandstones differ very 
widely in character, their value depending chiefly upon 
the nature of the cementing material which holds them 



330 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

together. In order that a stone may wear well and 
evenly in a pavement it is desirable that it be fine- 
grained, dense and homogeneous, as well as cemented 
by a material which is not brittle and is nearly imper- 
vious to moisture. Those sandstones in which the 
cementing material is of an argillaceous or calcareous 
nature are apt to be perishable when exposed to the 
weather. The Medina sandstones of Western New 
York and Ohio have been quite extensively used for 
paving purposes and prove a very satisfactory material 
for such use. 

Limestone has not usually been successful in use for 
the construction of block pavements on account of its 
lack of durability against atmospheric influences. The 
action of frost commonly causes weakness and shiver- 
ing, which produces uneven and destructive wear under 
traffic. There are, however, as wide variations in the 
characteristics of limestones as in those of sandstones, 
and there may be possible exceptions to the rule that, 
in general, limestone is not a desirable material for 
block pavement. 

Art. 8i. Cobblestone Pavements. 

Cobblestones have in the past been quite extensively 
used in the construction of street pavements, although 
at the present time they have been for the most part 
abandoned. They are not usually durable pavements 
as the stones are easily loosened from their positions, 
although the stones themselves ma^^ be practically 
indestructible and used again and again in reconstruct- 
ing the surface 

Cobblestone pavements as commonh^ constructed 
are also objectionable because they are permeable to 



STONE-BLOCK PAVEMENTS. 33 1 

water and difficult to clean. They therefore collect, 
and become saturated with, the filth of the street 
and are very liable to injury from frost. They are 
also extremely rough and unsatisfactory in use for 
travel. 

For paving the side-gutters, where broken stone or 
sometimes where wood is used for the traveled portion 
of the street, cobblestones may often be convenient 
and useful, and form a cheap and satisfactory means 
of disposing of surface drainage. Such an arrange- 
ment is shown in Fig. t,^ (P- 365). 

Cobblestones as used for pavements are usually 
rounded pebbles from 3 to 8 inches in diameter. They 
are set on end in a la3^er of sand or gravel, rammed into 
place until firmly held in position, and then covered 
with sand or fine gravel and left to the action of travel, 
which soon works the upper layer of sand into the 
interstices between the stones. 

Art. 82. Belgian Blocks. 

Belgian block is the name commonh^ applied to a 
pavement formed of nearl^^ cubical blocks of hard rock. 
In the vicinity of New York this pavement has been 
largely used, the material being trap-rock from the 
valley of the lower Hudson. The blocks are usually 
from 5 to 7 inches upon the edges, with nearl3^ parallel 
faces, and as commonly laid are placed upon a founda- 
tion laj^er of sand or gravel about 6 inches thick. This 
shape of block is objectionable on account of the width 
between joints being too great to afford good foothold 
to horses. The materials of which Belgian blocks have 
ordinarily been formed are very hard and (as already 
noted in Art. 81) wear smooth in service, becoming 



332 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

slippery and thus increasing the effect of the too wide 
block. It is also better to have the length of the 
blocks somewhat greater across the street and let them 
break joints in that direction in order that they ma}^ 
give greater resistance to displacement under passing 
wheel-loads. 

The older pavements of this character were usually 
placed upon a sand foundation. More recently this 
practice has, in the better class of work, been super- 
seded by a more solid construction, a concrete base 
being used. 

These pavements are now very little used, having 
given place to granite or sandstone blocks. 

Art. 83. Granite and Sandstone Blocks. 

For the construction of the better class of stone- 
block pavements, blocks of tough granite or sandstone 
are used, set, in the best work, upon a concrete base, 
although sometimes placed upon a foundation of sand 
or gravel. 

These pavements when well constructed are about. 
the most satisfactory means yet devised for providing 
for very heavy traffic, as they present a maximum, 
resistance to wear with a fairly good foothold for horses, 
and are much more agreeable in service than the 
old form of rough pavements. There is still much to 
be desired in the attainment of smoothness and ab- 
sence of noise, and, as a general thing, it may be said 
that pavements of this kind are desirable only where 
the weight of traffic is so great that the smoother pave- 
ments would not offer sufficient resistance to w^ear. 
Even in such cases it may frequently be questionable 
whether an additional expense for maintaining a pave- 



STONE-BLOCK PAVEMENTS. 333 

ment which would be more pleasant in use and less 
objectionable to occupants of adjoining premises would 
not be advisable from an economical as well as from 
an aesthetic point of view. 

Blocks for stone pavements, in the best work, are cut 
in the form of parallelopipeds, 9 to 12 inches long, 3 
inches wide, and 6 or 7 inches deep. The length should 
be sufficient to permit the blocks to break joints across 
the street. The width should be less than that of a 
horse's hoof in order that the joints in the direction of 
travel may be close enough together to prevent a horse 
from slipping in getting a foothold. The depth should 
be sufficient to give a bearing surface in the joints 
large enough to prevent the blocks from tipping when 
the load comes upon one end of them. 

Specifications for granite blocks in New York City 
in 1908 are as follows: 

''The blocks to be used shall be of a durable, sound 
and uniform qualit3^ of granite, each stone measuring 
not less than eight (8) inches, nor more than twelve (12) 
inches in length; not less than three and one-half (3^) 
nor more than four and one-half (4^) inches in w^idth, 
and not less than seven (7) nor more than eight (8) 
inches in depth, and the stones shall be of the same 
quality as to hardness, color and grain. No outcrop, 
soft, brittle or laminated stone will be accepted. The 
blocks are to be rectangular on top and sides, uniform 
in thickness, to lay closeh^ and with fair and true 
surfaces, free from bunches. Over special construc- 
tions, the blocks may be of dimensions other than 
above specified when approved by the Engineer, The 
stone from each quarry shall be piled and laid separately 
in different sections of the work, and in no case shall the 
stones from different quarries be mixed. ■" 



334 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Art. 84. Construction of Stone-block 
Pavements. 

Stone-block pavement for durable and effective 
service should be placed upon very firm foundations. 
Bases of concrete are usually employed and give the 
best results. These foundations are formed as described 
in Art. S7- and consist of a layer of concrete 4 to 8 
inches thick, 6 inches being the most common depth. 

In constructing the pavement, a cushion coat of sand, 
usually I to 2 inches thick, is spread upon the base of 
concrete for the purpose of allowing the bases of the 
paving blocks to be firmly bedded when the tops are 
brought to an even surface, the sand readily adjusting 
itself so as to fill all the spaces beneath the blocks and 
to offer a uniform resistance to downward motion in 
every part of the pavement, and in like manner trans- 
mitting the loads which come upon the pavement to 
the foundation so as to evenly distribute them over the 
surface of the concrete. The sand used for this pur- 
pose should be clean and dry, and all large particles 
sifted out, as they may prevent the blocks adjusting 
themselves properly. A thin layer of asphaltic cement 
is sometimes used in place of the sand with very good 
results. 

The blocks should be laid as close together as pos- 
sible in order to make the joints small. They are laid, 
like brick, with the longest dimension across the street, 
and arranged in courses transverse to the street, with 
the stone in consecutive courses breaking joints. 

After the blocks are placed they are well rammed to 
a firm unyielding bearing and an even surface. Stones 
that sink too low under the ramming must be taken 
out and raised by putting more sand undemeatho 



STONE-BLOCK PAVEMENTS. 33S 

As in the case of other block pavements, those of 
stone should be made as impervious to moisture as 
possible. The foundation should be kept dry, and 
moisture prevented from penetrating beneath the 
blocks where it has a tendency to cause unequal settle- 
ment under loads or disruptions under the action of 
frost. In the better class of work, therefore, the joints 
are filled with an impervious material which cements 
the blocks together. Asphalt or coal-tar paving cement 
is commonly emploj^ed for this purpose, as with brick 
and wood, and seems the most satisfactory in use, 
although h^^draulic cement mortar is sometimes used. 
The coal-tar cement is commonly made by mixing 
coal-tar pitch with gas-tar and oil of creosote, a pro- 
portion sometimes employed being 1 00 pounds pitch, 
4 gallons tar, and I gallon creosote. 

The use of cement between the blocks binds them 
together and increases the strength of the pavement as 
well as the resistance of the blocks to being forced out 
of surface. It also deadens to some extent the noise 
from the passing of vehicles where asphaltic or coal-tar 
cement is used. 

A method commonly used for filling the joints is to 
first fill them about one third full of small gravel, then 
pour in the paving cement until it stands above the 
gravel; then another third full of gravel, more cement 
as before; then gravel to a little below the top, and the 
joint filled full of cement; after which a coating of 
fine gravel is distributed over the surface. 

Sometimes the joints are filled with gravel before the 
blocks are rammed to surface, and the paving cement 
afterward poured into the joints. This has the advan- 
tage of bringing the blocks to a very firm bearing, and 
secures complete filling of the joints. 



336 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Various modifications of the method above outlined 
are used in the principal cities for a pavement to with- 
stand heaviest traffic and secure a maximum of dura- 
bility; essentially it represents the best modern practice. 
The specifications used in New York City in 1908 
contain the following requirements: 

'' 29. On the concrete foundation, as designated, shall 
be laid a bed of clean, coarse, dry sand to such depth 
(in no case less th^n one and a half [i^j inches) as may 
be necessary to bring the surface of the pavement, 
when thoroughly rammed, to the proper grade. 

"'On this sand bed, and to the grade and crown 
specified, shall be laid the stone blocks at right angles 
to the line of the street or at such angle as may be 
directed. Each course of blocks shall be laid straight 
and regularly, with the end joints by a lap of at least 
three (3) inches, and in no case shall stone of different 
width be laid in the same course except on curbs. All 
joints shall be close joints except that when gravel 
filling is used, the joints between courses shall be not 
more than three-quarters (|) of an inch in width. 

''After the blocks are laid on a concrete foundation, 
they shall be covered with a clean, hard and dry gravel, 
which shall have been artificiall3^ heated and dried in 
proper appliances, placed in close proximity to the 
work, the gravel to be brushed in until all the joints are 
filled therewith to within three (3) inches of the top. 
The gravel must be washed white quartz and be entirelj^ 
free from sand or dirt, and must have passed through 
a sieve of five-eighths (f) inch mesh and been retained 
by a three-eighths (|) inch mesh. 

"The blocks must then be thoroughl}^ rammed and 
the ramming repeated until the3^ are brought to an 
unyielding bearing with a uniform surface, true to the 



STONE-BLOCK PAVEMENTS. 337 

given grade and crown. A^o ramming shall be done 
within twenty (20) feet of the face of the work that is 
being laid. 

"The boiling paving cement, heated to a temperature 
of 300° F.and of the composition hereinbefore described, 
shall then be poured into the joints until the same are 
full, and remain full to the top of the gravel. Hot 
gravel shall then be poured along the joints until they 
are full flush with the top of the blocks, when they 
shall again be poured with the paving cement till all 
voids are completely filled/' 

Art. 85. Stone Trackways. 

In some of the European cities, particularly in Italy, 
stone trackwaj^s are sometimes emplo^'ed on streets of 
heavy traffic for the purpose of diminishing traction. 
These trackways are formed of smooth blocks of stone 




Fig. 27. 

4 to 6 feet long, 18 to 24 inches wide, and 6 to 8 inches 
deep, laid flat and end to end so as to form a smooth 
surface upon which wheels may move with the least 
possible resistance. Between the tracks, and usually 
the remainder of the street, is commonly paved with 
cobble. The method of construction is shown in Fig. 
27. The tracks dram to the middle, and the pavement 
between is made concave and provided with openings 
into the storm sewers for the escape of surface-water. 
The track and pavement are laid upon a la\^er of sand 
resting upon a broken-stone or gravel foundation. 



333 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Such trackways are quite durable under heavy traffic, 
and give light tractive resistance. They are not, how= 
ever, desirable on the streets of towns where smooth 
pavements might be used, and are too expensive for 
use on country roads. 

Steel trackways have frequently been proposed, and 
in a few instances have been tried, but have not been 
found successful and do not seem likely to become of 
any considerable importance. 



CHAPTER XII. 
CONCRETE PAVEMENTS. 

Art. 86. Concrete as Surface Material. 

The use of Portland cement concrete as material for 
the surfaces of street pavements has been in use in a 
small way since about 1895. Until after 1900, however, 
these pavements were very few in number and regarded 
as rather doubtful experiments. Since 1900 there has 
been a considerable increase in the use of this material, 
and quite a number of cities have tried it to some extent. 
Most of the work that has been done is of too recent date 
to show final results, or determine the best methods of 
construction. In several instances, the early pavements 
have given good wear under moderate traffic, but more 
experience is necessary to determine the extent to which 
these materials may meet the requirements of more general 
use, and to formulate methods of construction to secure 
the best results. 

The objects in most instances of engineers who have 
constructed pavements of this kind have been to secure 
pavements for moderate or light traffic at less cost than 
brick, or other satisfactory pavements, could be con- 
structed. Several methods of construction have been 
patented, and many, if not most, of the concrete pave- 
ments now in use have been constructed under some of 
these patents. These refer both to the composition of 
the cement mortar or concrete employed for surfacing 
the pavement, and to the method of construction. 

339 



340 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Three types of construction have been used for pave- 
ments of this class: 

(a) Mortar-surfaced pavements, in which a surfacing 
of mortar is applied to an ordinary concrete foundation 
before the concrete has set, in order that adhesion may 
develop between the mortar and concrete in setting. 

(b) Monolithic concrete pavements, which consist 
of a single layer of concrete of the full thickness of the 
pavement. 

(c) Grouted concrete pavements, constructed by first 
placing the coarse aggregate to the required thickness, 
and then pouring a grout of cement mortar over the 
surface, so as to fill the voids in the aggregate, which is 
rolled to a firm surface either before or after the grout 
is appHed. 

Concrete pavements seem to give promise of consider- 
able development in the immediate future, and it is prob- 
able that their use will rapidly extend. 

Art. 87. Portland Cement. 

Portland cement is manufactured by burning a mixture 
of limestone and clay, shale or other argillaceous mate- 
rial; the mixture being accurately proportioned to give 
correct relations between the percentages of lime, silica 
and alumina in the resulting cement. The materials 
are finely ground, so as to obtain an intimate mixture, 
and the temperature of burning carefully regulated to 
secure proper chemical combinations. The clinker ob- 
tained from the burning of these materials is then finely 
ground into the powder which is used as cement. 

The manufacture of Portland cement is very exten- 
sively carried on throughout the United States, and 
cement of high grade can be obtained with little difficulty 



CONCRETE PAVEMENTS. 341 

in nearly any locality. It is important that only first- 
class cement should be used in the construction of paved 
surfaces, and care should always be taken in the selec- 
tion of the cement. The requirements of this service 
are very severe, and the soundness of the cement is of 
special importance. When it is not feasible to make 
tests of the cement, considerable reliance may be placed 
upon the selection of a good brand, but usually tests are 
imposed to insure proper quality. 

The tests upon which dependence is placed to form 
judgment of the quality of cement are those for tensile 
strength, fineness and soundness. The standard methods 
of testing cement, which have been recommended by the 
American Society of Civil Engineers are now commonly 
employed, and standard specification requirements rec- 
ommended by the American Society for Testing Materials 
are very generally followed. These requirements are 
as follows : 

PORTLAND CEMENT. 

" Definition. This term is applied to the finely 
pulverized product resulting from the calcination to 
incipient fusion of an intimate mixture of properly pro- 
portioned argillaceous and calcareous materials, and 
to which no addition greater than 3 per cent has been 
made subsequent to calcination. 

SPECIFIC GRAVITY. 

" The specific gravity of cement shall not be less than 
3.10. Should the test of cement as received fall below 
this requirement, a second test may be made upon a 
sample ignited at a low red heat. The loss in weight 
of the ignited cement shall not exceed 4 per cent. 



342 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

FINENESS. 

'' It shall leave by weight a residue of not more than 
8 per cent on the No. loo, and not more than 25 per cent 
on the No. 200 sieve. 



TIME OF SETTING. 

^' It shall not develop initial set in less than 30 minutes; 
and must develop hard set in not less than one hour, 
nor more than 10 hours. 



TENSILE STRENGTH. 

'' The minimum requirements for tensile strength for 
briquettes one square inch in cross-section shall be as 
follows, and the cement shall show no retrogression in 
strength within the periods specified: 

Neat Cement. 

Age. Strength. 

24 hours in moist air 175 lbs. 

7 days (i day in moist air, 6 days in water) 500 * ' 

28 days (i day in moist air, 27 days in water) 600 " 

One Part Cement, Theee Parts Standard Ottawa Sand, 

7 days (i day in moist air, 6 days in water) 200 lbs. 

28 days (i day in moist air, 27 days in water) 275 " 

CONSTANCY OF VOLUME. 

" Pats of neat cement about 3 inches in diameter, 
J inch thick at the center, and tapering to a thin edge, 
shall be kept in moist air for a period of 24 hours. 

" (a) A pat is then kept in air at normal temperature 
and observed at intervals for at least 28 days. 



CONCRETE PAVEMENTS. 343 

" {b) Another pat is kept in water maintained as 
near 70° F. as practicable, and observ^ed at intervals for 
at least 28 days. 

" (6) A third pat is exposed in any convenient way in 
an atmosphere of steam, above boiling water, in a loosely 
closed vessel for 5 hours, 

" These pats, to satisfactorily pass the requirements, 
shall remain firm and hard, and show no signs of dis- 
tortion; checking cracking or disintegrating. 

SULPHURIC ACID AND MAGNESIA. 

" The cement shall not contain more than 1.75 per 
cent of anhydrous sulphuric acid (SO3), nor more than 4 
per cent of magnesia (MgO)." 

Portland cement is sold packed in barrels, containing 
about 376 pounds of cement, or in canvas bags, each 
containing one-fourth barrel. For the purpose of pro- 
portioning mortar or concrete by volume a barrel is com- 
monly taken as holding ^.S cubic feet, or the cement is 
assumed to weigh about 100 pounds per cubic foot. 
Cement must always be stored in a dry place and care- 
fully protected from moisture; it should not be piled in 
contact with the ground, and must be covered to exclude 
all water. 

Art. 88. Portland Cement Mortar. 

Portland cement mortar is formed by mixing Portland 
cement with a fine mineral aggregate, and wetting the 
mixture to a paste, which hardens with the setting of the 
cement. In the preparation of cement mortar, care 
must be used to insure that the cement and aggregate 
are thoroughly mixed and evenly distributed through the 
mortar. WHien mixing by hand, the dry materials should 



344 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

be mixed to a uniform color before adding the water. 
The mortar must in all cases be placed in the work 
before beginning to set, and be left undisturbed until 
thoroughly set, and until it has acquired sufficient strength 
to resist distortion. 

Fine Aggregate. The fine aggregate is either sand, 
stone screenings or other similar material, which will 
pass a screen of -J-inch mesh. Sand for this use should 
be coarse or contain a considerable perceatage of coarse 
grains (not passing a sieve of thirty meshes to the inch). 
It should be clean and free from loam, and other impur- 
ities, and should not contain more than from 3 per cent 
to 6 per cent of dust which will pass a sieve of 100 meshes 
to the inch. 

In preparing mortar, good results require that the voids 
in the fine aggregate should be completely filled with 
cement, and the amount of cement used should be 
somewhat larger than the volume of voids to be filled. 
It is therefore desirable that the sizes of the particles of 
fine aggregate be graded so as to give a minimum of 
voids, and, in important work it is necessary to carefully 
examine the aggregate in this particular and proportion 
the mortar accordingly. The following tests for deter- 
mining the voids in aggregates, recommended by a com- 
mittee of the National Association of Cement Users, 
may be used with advantage for this purpose: 

Test for Voids. To determine the voids in the coarse 
aggregate or fine aggregate: Prepare a vessel, the cubical 
contents of which is exactly one cubic foot (1728 cubic 
inches), being smaller at the top than at the bottom. 
Fill the vessel with the aggregate, thoroughly dried, 
* coarse ' or ' fine ' as the case may be, which is to be used. 
Shake or jar the vessel containing the aggregate until it is 
compacted as thoroughly as possible and the vessel is 



CONCRETE PAVEMENTS. 345 

level full. Then ascertain the net weight of the fine 
aggregate in the vessel, deduct this weight from w (the 
weight of one foot cube of mineral of which the fine 
aggregate is composed) divide the difference thus obtained 
by w. The result is the percentage of voids. 

When mortar surfaces are employed upon pavements, 
the fine aggregates are frequently composed of crushed 
granite screenings, or sometimes of special mixtures of 
granite and sand, or limestone, intended to give effective 
resistance to wear, or to render the paving surface gritty 
and prevent it being slippery. Some of these com- 
binations of materials are patented. For the purpose 
of comparing the values of various fine aggregates for 
use in mortars, the Committee of the National Associa- 
tion of Cement Users recommends that they be tested 
in comparison with standard sand, as follows: 

" Mortars composed of i part Portland cement and 
3 parts fine aggregate by weight when made into briquettes 
should show a tensile strength of at least 70 per cent of 
the strength of i : 3 mortar of the same consistency made 
with the same cement and standard Ottawa sand. To 
avoid the removal of any coating on the grains which 
may affect the strength, bank sands should not be dried 
before being made into mortar, but should contain 
natural moisture. The percentage of moisture may be 
determined upon a separate sample for correcting weight 
of sand. From 10 to 40 per cent more water may be 
required in mixing bank or artificial sands than for 
standard Ottawa sand to produce the same consistency." 

Art. 89. Portland Cement Concrete. 

Portland cement concrete consists of a mixture of 
Portland cement, or Portland cement mortar, with a 



346 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

coarse aggregate (gravel, broken stone or similar material), 
the materials being so proportioned as that the mortar 
fills the voids in the coarse aggregate, and the cement 
those of the fine aggregate. 

Coarse Aggregate. The coarse aggregate usually con- 
sists of gravel or crushed rock, all of which is retained by 
a sieve of J-inch meshes, and grading upwards to any 
size which may be readily handled in mixing and not 
too large to pack in the thickness of the concrete layer. 
The largest stones should not be greater in diameter than 
f the depth of the concrete layer, and a grading of sizes 
is desirable, which will reduce the voids to be filled with 
mortar to a minimum. In some instances, the aggregate 
is screened into several sizes, and these mixed in such 
proportions as to give a minimum of voids. 

When the concrete is to be used as surface material 
care should be taken to secure a uniform mixture, in 
order that the resistance to wear may be even. Some 
engineers specify nearly uniform sizes of rock on this 
account, using a larger proportion of mortar. Commonly, 
however, crusher-run rock is used, with only the fine 
parts and the pieces which are too large screened out, 
no grading of sizes being attempted on account of cost. 

The material used for the coarse aggregate should be 
of hard and durable character. Gravels, limestones 
and trap rocks are commonly employed. For the mono- 
lithic pavement, where the concrete forms the wearing 
surface of the pavement, particular attention should 
be given to securing rock that will be resistant to wear. 
For such use, special mixtures of rock are sometimes 
made, two kinds of rocks of somewhat different wearing 
properties being used in fixed proportions. Some of 
these mixtures are patented. In general the choice of 
stone is limited by what may be available in the neigh- 



CONCRETE PAVEMENTS. 347 

borhood of the work, but careful consideration should 
always be given to the suitability of the material. When 
the concrete is for foundation only and not exposed upon 
the surface, a poorer grade of rock may be employed. 
The ^Association for Standardizing Paving Specifica- 
tions has suggested the following specification for con- 
crete for use in pavements : 

FINE AGGREGATE. 

" The fine aggregate shall consist of any material of 
siliceous, granite or igneous origin, free from mica in 
excess of 5 per cent, and other impurities, uniformly 
graded, the particles ranging in size from ^ inch down to 
that which will pass a No. 100 standard sieve. 

COARSE AGGREGATE. 

" The coarse aggregate shall be sound broken stone, 
trap rock, or granite having a specific gravity of not less 
than 2.6. It shall be free from all foreign matter, uni- 
formly graded and shall range in size from I inch up, 
the largest particles not to exceed in any dimension one- 
half the thickness of the concrete in place. 



PROPORTIONS. 

" In preparing the concrete the cement and aggregate 
shall be measured separately, and then mixed in such 
proportions that the resulting concrete shall contain 
fine aggregate amounting to one-half of the volume of 
the coarse aggregate, and that five cubic feet of concrete 
in place will contain ninety-four (94) pounds of cement. 



348 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

MIXING. 

" The ingredients of the concrete shall be thoroughly 
mixed, sufficient water being added to obtain the desired 
consistency, and the mixing continued until the materials 
are uniformly distributed and each particle of the fine 
aggregate is thoroughly coated with cement, and each 
particle of the coarse aggregate is thoroughly coated with 
mortar. 

'' Where a mechanical mixer is used, the materials must 
be proportioned dry, and then deposited in the' mixer 
all at the same time. The mixer must produce a con- 
crete of uniform consistency and color, with the stones 
thoroughly mixed with water, sand and cement. 

CONSISTENCY. 

" The materials shall be mixed wet enough to produce 
a concrete of a consistency that will flush readily under 
light tamping, but which can be handled without causing 
a separation of the coarse aggregate from the mortar. 

RE-TEMPERING. 

'' Re-tempering, that is, re-mixing with additional 
water, mortar or concrete, that has partially hardened, 
will not be permitted." 

Art. 90. MoRTAR-suRiACED Pavements. 

Most of the earlier concrete pavements are of this 
type. They consist of a concrete foundation, constructed 
in about the same manner as for a brick or asphalt sur- 
face, with a layer of carefully prepared cement mortar 



CONCRETE PAVEMENTS. 349 

to take the wear of the traffic. In many of the pav^ements, 
the method of construction is about the same as that 
used in placing cement sidewalks, the depth being usually 
somewhat greater, with a difference in finishing the sur- 
face to prevent slipperiness. 

The fine aggregate used in making mortar for this 
purpose should be of hard material, capable of resisting 
abrasion. Crushed granite and trap rock have fre- 
quently been employed for the purpose, and mixtures 
of granite and limestone, or natural sand and crushed 
limestone, are sometimes used. Fairly good results 
have been obtained with all of these materials. These 
pavements are commonly from 5 to 7 inches in depth, 
with a surface layer i^y or 2 inches thick. The mortar 
for the surface must always be placed before the founda- 
tion concrete has set in order to secure proper adhesion 
between the two. 

In finishing the surfaces of these pavements, some 
method, intended to prevent the surface being too smooth 
and slippery, has usually been adopted. Sometimes the 
surface is cat into small blocks, tool cuts are made across 
the street a few inches apart, or the surface is pitted wnth a 
brass roller. In other instances, brushing the surface 
with a stiff broom has seemed to leave the surface in good 
condition for wear, while the use of aggregate composed 
of a mixture of two materials of somewhat different 
wearing qualities has been claimed to assist in prevent- 
ing the pavement being slippery. 

Expansion joints along the curb, or between the gutter 
and roadway proper, and also extending across the street 
at frequent intervals are needed to prevent the cracking 
of the pavement with the expansion and contraction of 
the pavement. These joints extend through the pave- 
ment and are usually filled with bituminous cement 



350 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

filler, although creosoted wood blocks are being used to 
some extent for the transverse joints. In the earlier 
pavements expansion was taken care of by dividing the 
pavement into rectangular blocks, as in the construction 
of sidewalks, but it was found that the wear of the pave- 
ments was mainly due to the breaking down at these 
joints, and particularly the longitudinal joints, and later 
practice has for the most part abandoned all continuous 
longitudinal joints, except at the sides. A method of 
construction which has been somewhat extensively used 
for this type of pavement, is patented and is known as 
the Blome Granitoid Pavement. Specifications some- 
times used in the construction of surface of this pavement 
are as follows: 



MIXING AND LAYING OF CONCRETE AND FORMATION 
OF THE BLOME COMPANY GRANITOID BLOCKING. 

The concrete and blocking hereinafter specified shall 
be constructed and manipulated according to the Blome 
Company patents and processes, using materials mixed 
in the proportions and laid as herinafter specified. 

The pavement shall consist of 5I inches of concrete, 
and surface blocking if inches, making a total of 7 inches, 
exclusive of foundation. 

After the sub-grade and foundation have been prepared 
as hereinbefore specified, there shall be deposited con- 
crete composed of i part of Portland cement, 3 parts sand, 
and 4 parts of crushed limestone, trap rock, or clean 
gravel. These materials to comply with the require- 
ments hereinbefore set forth and shall be mixed by special 
mixing machine suitable for the purpose to be approved 
by the engineer and shall be mixed at least five times 
before being removed from the mixer. The concrete 



CONCRETE PAVEMENTS. 35 1 

shall be thoroughly tamped in place, and shall be 5J 
inches thick, uniformly at all points, after having been 
compacted, shall be laid in sections with expansion 
joints, all as per the Blome Company patents and shall 
follow the slopes of the finished pavement so that the 
surface blocking is and shall be uniformly of the same 
thickness at all points. 

Surfacing Material. After the concrete has been 
placed and before it has begun to set, there shall be imme- 
diately deposited thereon the Granitoid Blocking which 
shall be if inches in thickness to be composed of two 
parts of the hereinbefore specified Portland cement and 
three parts of clean, crushed granite, trap rock, hard 
stone, crushed gravel, crushed boulders, or other sim- 
ilarily hard materials shall be screened with all the dust 
removed therefrom, utilizing the following composition 
of this material. 

Fifty per cent of the granite, trap rock, hard stone, 
crushed gravel, crushed boulders or other similarly hard 
materials to be what is known as ^-inch size, 30 per cent 
of the |-inch size, and 20 per cent of the re-'nch size 
with all finer particles removed. These proportions 
of sizes are extremely essential and must be kept abso- 
lutely accurate as in this lies one of the essential require- 
ments to produce proper results. This material to be 
mixed with cement thoroughly and after being wetted 
to a proper consistency and deposited on the concrete 
shall be worked into brick shapes of approximately 4J 
inches by 9 inches with rectangular surface similar to 
paving blocks, all as per special method and utilizing 
grooving apparatus as employed under the Blome Com- 
pany patents. The pavement shall be sloped in a manner 
as required by the city engineer. 

Should there by any part or parts of this pavement 



352 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

when completed where the slopes, contours, etc., have 
not been carried out in true manner then under this 
specification the contractor will be required to take up 
such part or parts down to the foundation and replace 
same to the proper level without expense of any kind to 
the city. 

Expansion Joints. The contractor for the work above 
specified shall also be required to provide for expansion 
joints across the pavement at such locations as may be 
necessary, which expansion joints shall extend through 
the blocking and concrete and shall be filled with a com- 
position especially prepared for the purpose according 
to the Blome Company patents. These expansion joints 
shall be constructed in an extremely careful manner under 
specific direction of the city engineer. 

For the purpose of diminishing the wear at the expan- 
sion joints, a method of reinforcement has been devised 
and patented, which protects the corners of concrete 
at the joints. This protection may be applied to any 
concrete surface. A pavement is constructed by the 
owners of this patent, known as the Baker Armored 
Concrete Pavement, specifications for surface and joints 
of which are as follows : 



WEARING SURFACE. 

'' After placing the above concrete base, and before 
it has taken its initial set, there shall be placed thereon 
a two (2) inch wearing surface of the following component 
parts : 

" One (i) part of cement to one and one-half (ij) parts 
of clean, sharp sand, and three (3) parts of hard head 
pebbles of a uniform size to be not less than one-quarter 
(J) inch in diameter, and not more than one-half (J) 



CONCRETE PAVEMENTS. 353 

inch in diameter. The proportion of sand to stone given 
is approximate as absorption tests may show the necessity 
of variation. Stone and sand to be screened and subjected 
to tests as described under paragraph No. 6 from time 
to time as the work progresses in order to insure a soUd 
stone with all the voids completely filled with sand and 
cement. 



ARMORED EXPANSION JOINT. 

" Expansion joints one-half (J) inch wide shall, be made 
wherever necessary to provide for expansion and con- 
traction. Expansion joints to be filled with No. 6 pitch 
or other approved material. At intervals of twenty-five 
(25) feet, expansion joints will be provided extending 
from curb to curb. 

" Where pavement comes in contact with street car 
or other tracks, expansion joints shall be made at the end 
of ties to provide for vibration caused by the jar of pass- 
ing cars. 

" All expansion joints are to be armored and sharp 
edges protected against abrasion by means of angles of 
•j-inch steel plates, 3 inches wide, provided with shear 
members which tie them securely to concrete base and 
wearing surface. These are clamped to a dividing board 
shaped to conform to the crown of the street. After 
pavement has been finished, the dividing board must be 
removed, and opening covered with tar paper until 
filled with No. 6 paving pitch or other specified material. 

" After the pavement has been finished for twenty- 
four (24) hours, it shall be covered with sand or shavings 
for a period of seven (7) days." 



354 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Art. 91. Monolithic Concrete Pavements. 

The construction of pavements of a single layer of 
concrete without the mortar surface seems to be to some 
extent superseding the mortar surface pavement, and to 
have given good results in a number of instances. It is 
claimed for this construction that the surface is not so 
apt to wear slippery, and that the larger aggregate offers 
greater resistance to wear than the mortar made of fine 
materials. The comparative value of the two methods in 
any particular case probably depends mainly upon the 
availability of suitable materials, and the cost of con- 
struction. The aggregate used in making concrete for 
this purpose should be hard, tough materials, capable 
of resisting abrasion. Hard limestones, graaites, or 
trap-rock may be used, and the concrete must be very 
thoroughly and uniformly mixed, and must be placed 
and compacted before beginning to set. The propor- 
tions of the materials in the concrete should be such 
that all voids are thoroughly filled. A mixture of two 
grades of rock of different wearing qualities has some- 
times been used for the purpose. A pavement, in which 
a mixture of limestone and granite is used, is patented, 
and known as the Moran Pavement. 

In finishing the surface of a pavement of this type, 
the concrete is tamped until the mortar flushes to the 
surface, which may then be broomed to give a finish, or 
marked by tamping lightly upon planks laid transversely 
upon it. These surfaces are not commonly grooved, 
but expansion joints are used, as with the mortar-surfaced 
pavements. It is usually, the effect of contraction rather 
than that of expansion, which needs to be provided for 
in such work, and this contraction may be to some extent 
obviated by covering the surface with sand and keeping 



CONCRETE PAVEMENTS. 355 

it moist for a considerable time after placing the pave- 
ment. 

The Association for Standardizing Paving Specifica- 
tions have suggested the following method for placing a 
concrete pavement: 

PLACING CONCRETE. 

" The concrete shall be deposited in a layer on the 
sub-grade in such quantities that, after being thoroughly 
rammed in place, it will be of the required thickness, 
and the upper surface shall be true and uniform. 

'' In conveying the concrete from the place of mixing 
to the place of deposit, the operation must be conducted 
in such a manner that no mortar will be lost and the 
concrete must be so handled that it will be of uniform 
composition throughout, shovring no excess or lack of 
mortar in any place. 

THICKNESS. 

"The thickness of the pavement shall be — inches, 
with its upper surface on the finished grade. 

" The minimum thickness for concrete pavement shall 
be 5J inches. 

FINISHING. 

" The pavement shall be finished by thorough hand 
tamping, until the mortar flushes freely to the surface, 
then lightly tamping with a template made of 2-inch 
plank shaped to conform to the curvature of the surface 
of the finished pavement and having a length of not less 
than one-half the width of the roadway to give a uniform 
surface with the slight markings thus made transverse 
to the street. 



356 A TEXT-BOOK ON ROADS AND PAVEMENTS. 
EXPANSION JOINTS. 

'' Expansion joints shall be placed at right angles to 
the curb line at intervals of 50 feet. These joints shall 
be not less than i inch wide and shall be filled with 
creosoted soft wood timber with the grain vertical and 
extending full depth of the pavement. 

PROTECTION TO WORK. 

" During the first four days after placing, the pavement 
shall be kept moist and it shall be protected against 
traffic until the concrete has thoroughly set. In no event 
shall the pas^ement be used within ten days after being 
laid." 

Art. 92. Grouted Concrete Pavement. 

Concrete pavements are sometimes constructed by 
placing a layer of broken stone upon the road-bed, roll- 
ing to a firm surface, and grouting with Portland cement 
grout, so as to fill the voids in the stone. 

A pavement of this kind is patented and known as the 
Hassam Pavement. In the construction of these pave- 
ments, a grouting of i to 2 Portland cement mortar is 
used after the stone has been firmly compacted by rolling 
and the voids reduced to a minimum. The rolling is 
continuous during the process of grouting. Upon the 
surface so obtained, a thin layer of pea stone is spread, 
grouted and rolled until the grout flushes to the surface. 

In constructing the Long Island Motor Parkway, a 
Hassam pavement was used, which was reinforced with 
wire fabric. In this case, a 2 J- inch layer of stone was 
placed, the wire fabric laid upon this, and another layer 
of stone of the same depth added. The stone was then 
rolled and the pavement finished as usual, no expansion 
joints being used. 



CHAPTER XIII. 

CITY STREETS. 

Art. 93. Arrangement of City Streets. 

The location of streets should be planned with a 
view to giving direct and easy communication between 
all parts of a city. The arrangement should also be 
such as to permit the subdivision of the area traversed 
by them in such a manner as to give the maximum of 
efficiency for business or residential purposes. The 
most obvious and satisfactory method of accomplishing 
these purposes is usually by the use of the rectangular 
system, with occasional diagonal streets along lines 
likely to be in the direction of considerable travel. 

Streets so far as possible should be systematically 
arranged and continuous throughout the extent of the 
city, both to facilitate travel and to admit of their being 
so named and numbered that the locality of a place 
of business or residence may at once be evident, from 
its address, to any one familiar with the general plan 
of the city. The rectangular system is desirable on 
this account, and also because it furnishes blocks of the 
best form for subdivision into building lots. 

The proper arrangement of streets will always neces- 
sarily depend in some measure upon the natural feat- 
ures of the locality, and any system of arrangement 
will be more or less modified by local topography. 
Where for topographic or aesthetic reasons it may be 
considered desirable to use curved lines for the streets, 

357 



358 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

the continuity and uniformity of arrangement should 
be maintained as far as possible. The use of curves 
on residence streets may sometimes be advantageous 
in reducing gradients or in its effect upon adjoining 
property through avoiding heavy earthwork. Where 
a change in direction is necessary the use of a curve 
usually gives a better appearance than an abrupt bend, 
unless the change can be effected at the intersection of 
a cross-street. Care is required, however, to prevent 
the local introduction of curvature disarranging the 
general plans and producing the chaotic condition due 
to an irregular use of short streets. 

In laying out a rectangular system of streets the 
blocks ordinarily will preferably be long and narrow. 
The distance needed between streets in one direction 
is only that necessary to the proper depth of lots, while 
in the other direction the streets need only be close 
enough to provide convenient communication for the 
travel and traffic. A convenient method would be to 
lay out the main streets so as to form squares large 
enough to permit the introduction of an intermediate 
minor street through the blocks. These minor streets 
may then be introduced in the direction that seems 
advisable in each locality. Such an arrangement is 
shown in Fig. zS. The diagonal streets cut more space 
from the blocks traversed by them, but give more 
frontage and property fronting them will usually have 
more value than other property in its vicinity. 

The proper location for diagonal streets intended as 
thoroughfares for traffic is naturally determined by the 
positions of the business centers or public buildings 
and parks, from which they may radiate in such manner 
as to bring the outlying portions of the city into the 
most direct communication possible. 



CITY STREETS. 



359 



A city cannot usually be laid out complete. Its for- 
mation is a matter of gradual growth and enlargement, 
and the end cannot be seen from the beginning. For 
this reason it is frequently necessary to undergo great 
expense in the larger cities in cutting new streets or in 
changing the positions or dimensions of existing old 
ones in built-up districts in order to relieve the 
crowded condition of the streets, which hampers busi- 
ness and renders travel difficult and unpleasant. Much 
of this difficulty might frequenth^ be obviated if in 




Fig. 



growing towns and cities proper attention were given 
to the regulation of suburban development. Such 
development should be under municipal control so 
far as to require at least that each new subdivision 
which opens new streets should be made with a view 
to affording proper ways of communication between 
adjoining properties by making streets continuous. 
Where such regulation does not exist streets will be 
laid in any manner to best develop the particular prop- 
erty in which the3^ are placed » 

A good example of the advantages of systematic and 
liberal plans in street arrangement, as well as of the 



360 A TEXT-BOOK ON ROADS AND PAVEMENTS. 




1 MMy!><r\ 






Fig. 29. 



CITY STREETS. 



361 



evils of unregulated extension, is given by the case of 
Washington, D. C. 

Fig. 29 shows a portion of the city of Washington 
illustrating its systematic arrangement. It consists of 
a rectangular system, together with two sets of diag- 
onal avenues, and open squares or circles at the inter- 
sections of the avenues. 

Fig. 30 shows a number of suburban subdivisions on 
the borders of the cit}^ of Washington, made previous 




Fig. 30. 

to the adoption of the law regulating them. In some 
cases the streets of adjoining subdivisions have no 
communication with each other, and the general ten- 
dency is toward a labyrinth of short streets. The law 
now requires that all street extension w^ithin the Dis- 
trict of Columbia shall conform to the general plan of 
the city of Washington; and under the operation of 
this law the lines of many of the city streets have been 
extended to all parts of the District, and all of the 
suburban development is being gradually brought with 



362 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

the city into one harmonious whole, on the same gen- 
erous plan that exists within the city. The rectification 
of the irregular plats upon the borders of the city must^ 
however, be a matter of heavy expense to the District. 

Art. 94. Width and Cross-section. 

The width of city streets is important both on 
account of its influence upon the ease with which 
traffic may be conducted, and because of its effect upon 
the health and comfort of the people, by determining 
the amount of light and air which may penetrate into 
thickly built-up districts. 

To properly accommodate the traffic of commercial 
thoroughfares in business districts of towns of consider- 
able size, it is desirable that a street should have a 
width of TOO to 160 feet, the whole of it to be used for 
roadway and sidewalks. Wide streets are especially 
needed where, as in the larger cities, they are bordered 
by high buildings or are to carry lines of street railway. 

Residence streets in a town of considerable size, 
where houses are set out to the property line and stand 
close together, should have a width of at least 80 to 
100 feet in order to look well and give plenty of light 
and air. 

The streets in nearly all large towns are laid out too 
narrow; they are crowded and dingy. The chief diffi- 
culty is that the future of a street is not usually fore- 
seen when it is located. Owners in subdividing prop- 
erty are only anxious to get as many lots as possible 
out of it, and there are usually no regulations looking 
to the future health and comfort of residents when the 
street shall be built upon. In the growth of a town the 
nature of localities changes; residence streets become 



CITY STREETS. 363 

business streets, streets devoted to retail trade become 
wholesale streets, and mercantile districts are given up 
to manufacturing. If a city could be laid out com- 
plete from the beginning it would be comparativeh^ 
eas3^ to consider the requirements to be met and locate 
the streets accordingly. Under existing conditions this 
is not possible, but a more liberal policy in planning 
streets would usually be found of advantage in any 
growth that ma3^ ensue. There is also very frequently 
an immediate financial advantage in the enhancement 
of values due to wide streets. A lot 100 feet deep on 
a street 80 feet wide will nearl37- always be of greater 
value than if the same lot be no feet deep and the 
street only 60 feet in width. 

In Washington, D. C, which probably has the best 
general system of anj^ x^merican cit}^, no new street can 
be located less than 90 feet in width, and avenues 
must be at least 120 feet wide. Intermediate streets, 
called places, 60 feet wide, are allowed within blocks, 
but full-width streets must be located not more than 
600 feet apart. The value of this liberal policy to the 
cit3^ of Washington is evident not onl\^ in the increased 
comfort of the people, but in its large growth as a 
residential city and the increased value of property 
in it. 

While it is advantageous to have the street wide be- 
tween building-lines, it is not necessary that the whole 
street width be used for pavements. The street pave- 
ment should be gauged in width b}^ the immediate 
necessities of the traffic w^hich is to pass over it. The 
pavement should be wide enough to easilj^ accommo- 
date the traffic, but any unnecessary width is a tax 
upon the community in the construction and mainte- 
nance of more pavement than should be required, and 



364 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

perhaps diminishes the length of street which may be 
improved with available funds. Thus, for a residence 
street in general a width of 30 to 35 feet between 
curbs is usually ample, with a foot-walk upon each side 
5 to 10 feet wide. The remainder of the street width 
should be made into lawns upon each side, with tree 
spaces between the sidewalk and roadway. 

Fig. 31 shows in partial section the arrangement of a 
90-foot residence street for moderate traffic. For resi- 
dence streets of lesser importance, where the travel 
is light and the street is only required to furnish 
facilities to meet the needs of its immediate locality, a 
less width of pavement may often be advantageously 



II. . - 33 Ft. -Ji S'*^ - -8 n.: X - - LAWN 17 R-. •' 

Fig. 31. 

used. A pavement 24 feet wide is sufficient to accom- 
modate a very considerable amount of light driving, 
and in many places, especially in the smaller towns 
where funds for effective improvement are obtained 
with difficulty, even less widths may be employed with 
the result of improving the streets both in appearance 
and usefulness. All that is really needed in such cases 
is room for teams to pass comfortably and to turn 
without difficulty. The narrowing of roadways on 
streets of fight traffic to what is really necessary may 
often make possible improvements which will turn a 
broad sea of mud into a narrow, hard roadway and a 
grass-plat. Fig. 32 shows the arrangement of a village 
street 50 feet wide for light service. 

In many cases for village streets, where the traffic is 
light and it is essential that the cost of construction be 



CITY STREETS. 365 

low, it may be good practice to construct the traveled 
portion of the roadway of macadam, or other pave- 
ment, and use cobble gutters at the sides without 

Fig. 32. 

curbs. Fig. S3 shows a roadway 30 feet wide, with 
macadam middle and cobble gutters. In Saginaw, 
Mich., this method has been followed, using either 
macadam or wood blocks for the middle portion, and 
in the report of City Engineer Roberts for 1893 it is 
recommended as economical and efficient. 

The cross-section of streets must be arranged with 
reference to proper surface drainage. The street is 




given a crown at the middle to throw the water into 
the gutters, and sidewalks usually have a sufficient in- 
clination toward the gutter to cause them to drain over 
the curb. The crown necessary to insure good drain- 
age in the roadway depends upon the nature of the 
covering, being less as the surface is more smooth and 
less permeable to water. For macadam roadways, it 
may vary from about 4V to ^\ of the width of the 
roadwaj^ For the various pavements, the required 
crowTi varies from about ^0 to j^q of the width, accord- 
ing to the smoothness of the surface and the permea- 
bility^ of the construction. For brick, asphalt, or 
wood-block surfaces, a crown of from -^\ to xJ(T of 



366 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

the width is commonly ample. Stone blocks may 
need slightly more; while, on streets of considerable 
longitudinal slope, the crown may be made somewhat 
lighter. 

The form of section is usually a convex curve, 
sometimes circular but more often parabolic, the para- 
bolic curve differing but slightly from the circular. 
This form is shown in Fig. 34. The distance of the 
Jp c B A 



Fig. 34. 

curved surface below the horizontal through the highest 
point is proportional to the square of the horizontal 
distance from the center. Thus, if the distance A —D 
be divided into 3 equal parts, the vertical distance from 
B to the curve is one -ninth and from C four-ninths of 
that at D. 

The street is usually made practically level across, 
the curbs and sidewalks at the two sides being given the 



^ 



Fig. 35. 

same elevation. The parking at the sides may have a 
slope between the sidewalk and the building-line when 
it is necessary or advantageous. Sometimes, on streets 
along a slope, expense may be saved or adjoining 
property benefited by placing the sidewalk at a dif- 
ferent elevation from that of the street, as shown in Fig. 
5, or by placing one curb lower than the other and 
moving the crown of the road to one side, as shown in 
Fig. 35. 



CITY STREETS. '367 

The surface drainage of alle3^s is secured either by 
forming the section as in a street, with a crown at the 
middle and gutters and curbs at the sides, or, as is com- 
monly preferable with narrow alleys, by placing the 
gutter at the middle and sloping the pavement from 
the sides to the center. Where the gutter is in the 
middle it is common to make the bottom of the gutter 
of a flagstone 15 to 18 inches wide. Fig. 36 show^s a 







Fig. 36. 

center-drained allej^ with block-stone pavement upon 
sand foundation. 

The form shown in Fig. 36 is also usuall3" emploj'ed 
where concrete pavement is used for alleys, as is quite 
common. This is desirable in manj^ instances on ac- 
count of the good drainage afforded, and the resistance 
of the material to dampness. 

Fig. 37 shows a side-drained cobble pavement for an 



^^^^j^ 



Fig. 37. 

alley. These have been extensiA'ely used in the past, 
being usuall}- placed upon sand foundation. They are 
gradually being replaced by brick or concrete pave- 
ments. 

Art. 95. Street Grades. 

The grades of city streets necessarily depend mainly 
upon the topography of the site. Wherever possible. 



368 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

it is desirable that grades be uniform, between cross- 
streets. 

In establishing grades for new streets through unim- 
proved property, they may usually be laid with refer- 
ence only to obtaining the most desirable gradients for 
the street within a proper limit of cost. But where 
improvements have already been made, and located 
with reference to the natural surface of the ground, it 
is frequently a matter of extreme difficulty to give a. 
desirable grade to the streets without injury to adjoin- 
ing properties. In such cases it becomes a question of 
how far individual interests shall be sacrificed to the 
general good. It may be said in this connection that 
adjustments to new grades are usually accomplished 
much more easily than would be anticipated, and when 
accomplished the possession of a desirable grade is 
of very considerable value to adjoining property. Too 
great timidity should not, therefore, be felt in regard 
to making necessary changes because of the fear of in- 
juring property in the locality. 

Where a grade if made continuous between inter- 
secting streets would be nearly level, it is frequently 
necessary to put a summit in the middle of the block 
and give a light gradient downward in each direction 
to the cross streets in order to provide for surface drain- 
age. The amount of slope necessary to provide for 
proper drainage depends upon the character of the sur- 
face and smoothness of the gutter. For a surface of earth 
or macadam the slope should not be less than about I 
in 100, and for paved streets from I in 200 to I in 250. 

In some cases it may be possible to give sufficient 
slope to gutters to carry off the surface-water b3^ mak- 
ing the gutter deeper at the ends than in the middle of 
the block without making a summit in the crown of 



CITY STREETS. 369 

the street. The curb in such case would be made 
level or of uniform gradient. 

It may frequently be necessary to consider the 
effect of grade in determining the character of pave- 
ment to be employed upon a street. Asphalt is com- 
monly limited to grades of 4 or 5 per cent, although 
some engineers use it on 6 or 7 per cent grades. Brick 
is commonly used on grades up to about 8 per cent, 
and in some places has given satisfactory service on 10 
per cent grades. Wide joints, about J inch, are some- 
times used in brick pavements on steep streets, in order 
to afford a better foothold for horses. This, however, 
in other instances appears to be unnecessary, provided 
the pavement is kept clean and in good condition. 

Wood blocks may safely be used on grades of 5 or 
6 per cent, while smooth stone blocks are employed in 
about the same manner as bricks, being if anything a 
little more slipper}^ than bricks. Stone blocks of some- 
what rough character are successfullj^ used in some 
instances on grades of 12 or 13 per cent. 

In a report on the streets of Duluth in 1890, Messrs. 
Rudolph Hering and Andrew Rosewater recommend 
for steep streets, in addition to the above, that brick 
may be used in which the tops are rounded, and that 
wood blocks for such use have their upper edges cham- 
fered on each side, or if round blocks be used, around 
the blocks. Subsequent experience has, however, 
seemed to indicate that, except in extreme cases, such 
special construction is not necessary. 

On the streets too steep for smooth pavement it is not 
unusual to pave part of the street width with a smooth 
pavement, like asphalt, and the remainder with stone 
blocks or some rough pavement for use in slippery 
weather. 



370 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

Art. 96. Street Intersections. 

At intersections the crown of the roadway pavement 
on each street should, if possible, be continuous to the 
center of intersection, in order to prevent vehicles on 
one street from, being subjected to the jar incident to 
passing over the gutter of the other. Where a storm- 
sewer is available into which the water from the gut- 
ters on the upper side can be emptied this is a simple 
matter, but where such sewers do not exist it requires 
the adoption of some special means of draining the 
gutters on the upper side. This may sometimes be 
accomplished by a culvert across the street, the gutters 
being somewhat depressed at the corners to bring the 
channel sufficiently low. In other cases, where the 
slope is sufficient, it is more satisfactory to construct 
an underground pipe-drain from the upper comer to 
some point in the gutter below the crossing. 

Where the rate of grade is such that it is feasible, it 
is desirable that the grade of both streets should be 
brought to a level at intersections. The top of the curb 
at the four comers should be at the same elevation, thus 
permitting the continuation of the full section of each 
roadway until they intersect. It is also desirable that 
the sidewalks at the corners be level; that is, the points 
a a in Fig. 38 should all be placed at the same eleva- 
tion, which will make the entire street section, includ- 
ing sidewalks, horizontal across the direction of travel 
on each street. 

On very steep slopes it may not be possible to flatten 
out the grade to a level in crossing transverse streets, 
and in such cases the elevations require study, and need 
to be carefully worked out for each particular case. In 
the report of Messrs. Rudolph Hering and Andrew 



CITY STREETS. 



371 



Rosewater upon the streets of Duluth, it is recom- 
mended that in all cases the grade shall be reduced to 
3 per cent between the curb lines of cross streets, and 
the grade of the curb reduced in all cases to 8 per cent 
for the width of the sidewalks of intersecting streets. 
This is to be considered the maximum allowable rate 
of transverse grade, and onl3^ to be employed in case of 
necessit}^. If in Fig. 38 the arrow represents the direc- 
tion of steep slope, and the street transverse to that 
direction has a roadway 40 feet wide with sidewalks 



a 



Fig. 38. 

10 feet wide, the above limits would permit the curb 
at c to be 1.2 feet lower than that at h, and admit of 
a fall of 0.8 foot in the curb line from a to 6 and from 
c to d. If both streets have the same grade and 
width the curb at the lowest corner would be 2.4 feet 
lower than at the highest corner. 

Sometimes, where the parallel streets in one direction 
follow the lines of greatest slope, and the cross streets 
are normal to them, the proper grades at intersections 
may be arranged by giving the streets along the slope 
a section similar to that shown in Fig. '^^2 throughout 
its length, thus permitting the street in the direction 



372 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

of slope to continue its grade across the intersection 
without altering at that point the side slope of the 
cross street. 

For a case of maximum slope this would make the 
section of the roadway of the cross street a plane sur- 
face sloping uniformly from the upper to the lower 
curb, or in Fig. 35 it would transfer the street crown to 
the upper curb. 

Art. 97. Footways. 

Footways are not required to bear the heavy loads 
which come upon the roadway pavement, but in streets 
of considerable travel are subjected to a continual 
abrading action, and for good service are required to 
be of a material which will resist abrasion well, of so 
uniform a texture as to wear evenly, and not hard 
enough to become smooth and slippery in use. 

A good sidewalk should always present an even sur- 
face, and therefore requires a firm foundation to resist 
the displacement of the blocks of which it may be com- 
posed. It must also be durable under atmospheric 
changes, and of material that may be easity cleaned. 
The materials commonly employed are gravel, wood, 
brick, asphalt, stone, and concrete. 

Gravel walks are the cheapest of footways where 
suitable material is available. They are constructed in 
a manner similar to that used for gravel roadways, and 
require that the bed of the walk be well drained, and 
that it be well compacted by rolling or ramming before 
the walk is placed upon it. The best gravel walks are 
usually built upon a base of rough stone. This base 
may be 6 or»8 inches thick, and forms a solid founda 
tion upon which the gravel surface may be placed and 



CITY STREETS. 373 

sustained against settling. Walks constructed in this 
manner are frequently used in city parks where the 
travel is considerable. On suburban roads gravel 
walks usually consist of a thin surface of gravel laid 
upon the earth-bed, and are replaced by some other 
surface when a more expensive construction can be 
afforded. Gutters are frequently necessary to protect 
the walk from the wash of surface-water, which other- 
wise very quickly destro3^s it. 

Wood is commonly used for walks in the form of 
planks which are laid on stringers, the planks being 
placed perpendicularl3^ to the direction of travel. It is 
comparatively short-lived, and requires considerable 
expenditure for repairs, as the material is perishable 
and also wears rapidly. 

Brick footway pavements have been extensively used 
for many years, and form, when well constructed, a very 
durable and satisfactory sidewalk. As commonly con- 
structed they consist of ordinar}^ hard-burned bricks 
laid fiat upon a la3^er of sand over the earth-bed. For 
light travel, pavements so constructed ma\^ last well 
and give good service; but they are apt to soon become 
uneven through the sinking of the bricks because of in- 
sufficient foundation. 

In constructing such a pavement the sand layer 
should be well compacted b}'' rolling or ramming be- 
fore setting the bricks, which should also be rammed 
to a firm and even bearing. To give satisfactor}^ re- 
sults a foundation of sand and gravel or broken stone 
should be formed 8 to 10 inches in thickness. In 
Washington a layer of gravel 4 inches thick and well 
compacted is used, with a layer of sand of the same 
thickness upon it to receive the surface. In formmg 
the pavements, the bricks are laid flat and as close as 



374 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

possible. The joints are filled with sand, usually by 
coating the surface with a layer of sand before ram- 
ming and after completion a second coating, which is 
allowed to remain a few days after admitting the 
travel to it. 

Care must be used in selecting brick for this purpose 
to get only hard-burned brick of uniform quality, 
in order that the resistance to wear may be even. 
The use of vitrified paving brick, as used for roadway 
pavement, would be of advantage on walks subjected 
to heavy wear. 

The use of a concrete foundation and setting the 
brick on edge and in mortar, after the manner of con- 
structing a roadway pavement, makes a very durable 
sidewalk under heavy travel. It is, however, some- 
what expensive, and usually a stone surface would be 
preferable where such expense is to be incurred. 

Footway pavements of a concrete in which coal-tar 
is the binding material have been widely used, but have 
not usually been satisfactory in use. As commonly 
constructed they wear rapidl3^ and soften, becoming 
very disagreeable in hot weather. Some pavements 
of this character have, however, shown fairly good 
service. 

Numerous methods have been proposed and tried 
for the construction of tar footwalks, differing from 
each other in the materials mixed with the tar to form 
the concrete, and in the manipulation of the process. 
Ashes mixed with sand and gravel are usually em- 
ployed, and sometimes clinkers from an iron foundry. 
A somewhat successful pavement of this class has a 
small amount of Portland cement mixed with the ashes 
and sand used in forming the concrete before the addi- 
tion of the tar. 



CITY STREETS, 375 

Asphalt footway pavements are formed either of as- 
phalt blocks or of a surface of sheet asphalt. Where 
blocks are used they are laid in the same manner as 
brick upon a foundation of sand or gravel. The 
blocks, or tiles as they are commonly called, are usu- 
ally made flat, about 8 inches square and 2 to 2 J inches 
thick. They are laid with their edges either at right 
angles to the street line or at an angle of 45° with 
the street line — usuall3^ at right angles, on account 
of greater ease in laying. 

Sheet-asphalt footways are laid in the same manner 
as an asphalt street pavement, the pavement, however, 
being given a less thickness. In Washington, D, C, 
these pavements are made about 3 inches thick, and 
constructed upon a bituminous base. Material re- 
moved from street pavements in re-surfacing is used 
for forming the surface material of the footway. 

In Europe rock asphalt is frequenth^ used for foot- 
ways. Asphalt mastic is commonly emplo\^ed, mixed 
with sand or gravel to give a wearing surface. The in- 
gredients are heated together and applied hot to a 
broken-stone or concrete foundation. In Europe hy- 
draulic cement concrete is used for the base, as in the 
driveways, A layer of 3 or 4 inches of concrete is em- 
plo3^ed, with a surface layer of rock asphalt or asphalt 
mastic and sand, i to f inch in thickness for ordinary 
work. 

Natural stone for footwalks is ordinarily used in the 
form of flagging. Where flagstones of proper size and 
good wearing qualities may be readily obtained, this 
kind of pavement, if well laid, makes a durable and 
satisfactory footwalk. Flagstones should be set upon 
a solid foundation and be firmly bedded so as to 
preserve an even surface. Thej^ should not be laid, as 



376 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

is common in many places, directly upon an earth-bed 
but should have a cushion layer of sand or of some 
porous material to prevent unequal settling under the 
action of frost. 

CONCRETE SIDEWALKS. 

Concrete pavements, when well constructed of 
good materials, make the most satisfactory of foot- 
ways. They form an even surface, quite agreeable in 
service, and are durable and economical where exposed 
to considerable travel. 

In the construction of a concrete sidewalk a base 
of cinders is usually employed, supporting a layer of 
rather meager concrete and a thin surface layer of 
cement mortar. The cinders are commonlj^ 4 to 8 
inches thick, 6 inches being ample for most walks, and 
4 inches being sufficient for walks in residence. districts 
of small travel, where the soil is firm. Care should be 
taken to insure the proper drainage of the base, so 
that water ma}^ not remain in the soil immediately 
under the walk, or stand in the cinders„ The cinders 
should be placed to proper depth and well camped 
with the upper surface parallel to the finished top of 
the pavement. 

The concrete base is usually 3 or 4 inches thick, and 
sometimes on streets of heavy traffic it is made 5 inches. 
The wearing coat is from | inch to I inch in thickness, 
depending upon the wear to which it is to be subjected. 
A concrete base 3 J inches thick, with a wearing sur- 
face ^ inch thick, makes a ver3^ satisfactor3^ walk for 
residence streets carrying moderate travel. 

The composition of the concrete base must depend 
largely upon the materials available in the localit3^ 
Either gravel or broken stone may be used, with or 



CITY STREETS. 377 

without sand, according to the character of the 
materials. A mixture of one part Portland cement, 
three parts sand, and six parts broken stone is corti' 
monly used. When good hmestone is available, a 
mixture of one part Portland cement to four parts 
broken stone, without sand, is found ver3^ satisfactory^ 
the stone being broken to pass a one inch screen and 
with onh^ the fine dust removed. When sand or gravel 
is used, it is important that it be clean, as any dirt is 
likely to work to the surface in tamping and prevent the 
proper adhesion of the surface la^^er. For the same 
reason, the concrete must not be mixed too wet, and it 
should be well compacted b}^ ramming. 

The wearing coat is composed of Portland cement 
mortar, one part cement to one or two parts sand or 
screenings. The amount of cement used should be 
sufficient to fill the voids in the sand but not greatly in 
excess, as the resistance to abrasion is lessened by 
excess of cement. The material for v.^earing coat should 
be either clean, hard sand, or screenings from the 
crushed stone. The screenings should have the very 
fine dust removed, and when from a good quality of 
rock are superior to most natural sands. The mortar 
is brought to a uniform surface by drawing a straight 
edge along the tops of the forms at the sides of the walk. 
The surface is then worked smooth and uniform with a 
float 'and finished with a plastering trowel. 

Joints should be left at intervals of 4 or 5 feet to 
prevent irregular cracks through contraction of the 
concrete. These joints need not all extend through 
the base of the walk, but at intervals of 3 or 4 joints 
one should extend through. The surface of a concrete 
w^alk should have a transverse slope of about l ineh 
to I foot to provide for proper surface drainage. For 



378 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

residence streets of moderate travel a width of four to 
six feet is commonly required. Four feet is incon- 
veniently narrow unless the street is very little used, 
while six feet is sufficient for a very considerable 
amount of travel. 

Art. 98. Curbs and Gutters. 

Curbs are usually set in the streets of towns at the 
sides of roadway pavements for the purpose of sus- 
taining and protecting the sidewalk or tree space, and 
of forming the side of the gutter. They are commonly 
formed of natural stone or concrete, but sometimes clay 
blocks are used. 

STONE CURBSo 

The curbs used in different places vary considerably 
in form and dimensions. Stone curbs vary from 4 to 
12 inches in width and from 8 to 24 inches in depth. 
They are usually employed from 3 to 6 feet in length 
and set with close joints. 

The depth must be sufficient to admit of their being 
firmly bedded, and to prevent overturning into the 
gutter. The front of the curb should be hammer- 
dressed to a depth greater than its exposure above the 
gutter, and the back deep enough to permit the side- 
walk pavement to fit close against it where the side- 
walk adjoins the curb. The ends of the blocks should 
also be dressed to the depth of exposure, and the part 
below the ground trimmed off so as to permit the 
dressed ends to come in contact when laid. 

Granite is usually considered the best material for 
curbs, although both sandstones and limestones are 
used in many places. In the vicinity of New York the 



CITY STREETS. 



379 



North River bluestone has proved a good material for 
the purpose. 

There .are various ways of setting the curb. The 
object should be to bed it firmly on a solid foundation. 




Fig. 39. 

The best method is to place a bed of concrete under it. 
This construction is shown in Fig. 39, which repre- 
sents the method used in setting granite curb in Wash- 




FiG. 40. 

ington, D. C. The curb is held firmly in place bj^ the 
concrete foundation, which joins it rigidly to the road- 
way pavement. 

Where the concrete foundation is not used under 



380 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

the curb a deeper curbstone is necessary, usually from 
18 to 24 inches in good work. Curbs are very com- 
monly set in the natural ground, the pavement coming 
against it on one side; but it is usually found advan- 
tageous to lay them upon a bed of gravel or broken 
stone, with gravel filled in the trench about them. 
The ordinary method of setting curbs is shown in 
Fig. 40. 

The Washington specifications for ordinary work 
require that a bed of gravel 4 inches deep be used 
under the curb, and that the trench be filled with 
gravel placed in layers 3 or 4 inches deep, each layer 
being thoroughly rammed before adding the next. 

CONCRETE CURBS. 

Concrete curbs are extensively used, and their use is 
rapidly increasing, particularly in those sections where 
suitable natural stone does not occur. These curbs 
consist of concrete built in place in plank forms, 
extending continuously along the street, occasional 
joints being introduced to prevent irregular cracking. 
The concrete is laid and surfaced in the same manner 
as in sidewalk work, all exposed faces being surfaced 
with mortar. For curbs upon streets of light travel 
concrete mixed in the same proportions as for sidewalks 
gives good service; but where the use is more severe, a 
richer mixture will afford more strength, and about one 
part cement, two parts sand, and four of broken stone 
are frequently employed. 

The curb usually extends to the bottom of the base 
of the pavement, the concrete base joining the curb and 
holding it in place. For residence streets the curb is 
commonly 5 or 6 inches thick, while upon heavy traffic 



CITY STREETS. 38 1 

Streets it may be made 8 or 10 inches thick. Where 
the traffic is very severe, concrete curbs are frequently 
reinforced, or faced, with steel at the edges for greater 
resistance to shocks. Several forms of patented rein- 
forcement are available for such use. 



BURNED CLAY CURBS. 

Burned clay curbs are sometimes employed in small 
towns where brick pavements are used. These are 
made in such small sizes and short lengths that they 
are difficult to set to good line and are easily displaced. 
They have not in general proved satisfactory. 

Curbs at street corners and driveways are commonly 
laid upon curves. On ordinary residence streets curves 
of 4 to 6 feet radius are usually employed, while 
upon wide streets with considerable traffic curves of 
8 to 12 feet radius are desirable. These are easily set 
for concrete curbing by having forms to fit the whole 
curve. For curves of 3 or 4 feet radius stone curbing 
may be cut in two pieces for a right-angled turn. 
With curves of larger radius more pieces must be used 
and the cutting and setting becomes more expensive. 

GUTTERS. 

Gutters are commonly formed of the same material 
as the roadwa}^ pavement, which is simply extended 
to the curb. 

In streets paved w^ith brick or granite blocks the 
gutter blocks are sometimes turned lengthwise of the 
street, as shown in Fig. 22, for the purpose of facilitat- 
ing the flow of water in the gutter. As already pointed 
out, however, this has the effect of making a continuous 



382 A TEXT-BOOK ON ROADS AND PAVEMENTS 

joint between the pavement and gutter^ and its utility 
seems doubtful. 

For streets paved with broken stone it is common to 
employ stone gutters, formed of cobblestones, of narrow 
flags laid lengthwise of the gutter, or sometimes of rec- 
tangular blocks. Such construction is shown in Fig. 40. 
On streets paved with wood these gutters may also be 
frequently employed with advantage, especially where 
for any reason the gutter is likely to be kept damp. 
In forming a cobble gutter the stones are usually set 
upon a layer of sand or gravel after the manner of 
forming a cobble pavement. They should be firmly 
bedded and form an even surface. 

Cobble gutters are often used on village streets 
where no curbs are set, and in such locations, where 
but slight expense is admissible, they are quite satis- 
factory if properly constructed. This method of con- 
struction is illustrated in Fig. 33. 

Sometimes in work of this kind a flagstone is used 
for the bottom of the gutter and the sides are formed of 
cobble. This is preferable as affording a more free 
channel for the flow of the surface drainage. 

To obtain satisfactory results it is always necessary 
that the foundation be of sufficient depth and well 
compacted in order to prevent the surface becoming 
uneven by the stones being forced downward into the 
road-bed in wet weather or through the action of frost. 
A layer of 6 to 10 inches of gravel or sand is usually 
required. 

Where flagstones are used to form the gutter, they 
should be 3 or 4 inches thick, 10 to 15 inches wide, as 
may be required, and about 3 feet long. Care ;is 
required in laying that they may have an even bed and 
be well supported by the foundation. 



CITY STREETS 383 

Gutters of bricks, or of stone blocks, are often used for 
streets upon which the roadway pavement is asphalt, 
on account of the liability of the asphalt being injured 
b}^ dampness. In this case the gutter is constructed 
by setting the bricks or blocks with their greatest 
length along the street. They are placed upon a bed 
of concrete, the same as is used for the foundation 
under the asphalt surface, and the joints are filled 
with h3'draulic cement mortar, as in constructing brick 
pavement. 

CONCRETE GUTTERS. 

Concrete gutters are quite commonly used on streets 
paved with macadam, and sometimes at the sides 
of asphalt streets. These are sometimes flat curved 
gutters, similar in form to the cobble gutter shown in 
Fig. 33, but more commonly they are used with con- 
crete curbs as combined curb and gutter. These 
consist simply of the concrete curb with a concrete 
gutter 18 to 30 inches wide attached and built together 
in one piece. This is usually placed, like a sidewalk, 
upon a layer of cinders or gravel, and is constructed in 
the same manner, the gutter, upon streets of moderate 
traffic, being usually about 5 or 6 inches thick. 

Where the street pavement is carefull}^ laid flush 
with the gutter, this makes a very satisfactory- gutter. 
Upon macadam streets considerable trouble is some- 
times experienced in keeping the surface of the macadam 
up to the level of the gutter, and in many instances, 
unless considerable care is taken both in construction 
and maintenance, a second gutter forms in the surface 
of the macadam next to the concrete gutter. This is 
due to the difficulty of properly compacting the mac- 
adam next to the gutter, as well as to the greater wear 



384 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

of the macadam and the effect of rain in washing it. 
To secure good results it is essential that the macadam 
be brought flush with the concrete gutter so that the 
water may readily pass into the gutter, and that it be 
well compacted so as to prevent subsequent settlement. 

Art. 99. Crossings. 

On streets paved with a smooth hard surface which 
is easily cleaned, such as brick or asphalt, special foot- 
way crossings are not usually required or desirable, 
unless the foot travel be very considerable. On other 
pavements, however, which are apt to be rough to 
walk upon or muddy in bad weather, as upon stone, 
or macadarn, footways of flagstones, brick, or concrete 
are commonly provided. 

Stone crossings consist of flagstones about 10 or 12 
inches wide laid in rows across the street, the rows 
being 6 or 8 inches apart and paved between with stone 
blocks set in the ordinary manner. The crossing- 
stones are 3 or 4 feet long, and at least 6 inches thick 
in order that they may not be broken by the traffic. 
They should be laid with close joints and firmly bedded 
upon the foundation. 

Brick crossings are usually constructed in the same 
manner as brick street pavements, being laid upon 
concrete base and having joints filled with cement 
mortar. They should be slightly crowned, so as to 
raise the crossing a little above the general level of the 
street and facilitate keeping them clean. These cross- 
ings are sometimes laid as double layer brick pavements 
with sand filled' joints, but in general the better grade 
of construction is but slightly more expensive and is 
much more durable in use. 



CITY STREETS. 385 

Concrete street crossings are placed in the same 
manner as concrete sidewalks. They should, like brick 
crossings, have a crown to aid in keeping them clean. 
Crossings, and sidewalks across alley openings or drive- 
ways, need to be somewhat heavier than ordinary 
sidewalks and are usuall3^ about 6 inches thick. It is 
common to cut longitudinal V-shaped grooves, about 
i inch to I inch wide, i to ^ inch deep, and 4 inches 
apart, in the surface of the walk to afford a foothold to 
horses in crossing it. These grooves may readily be 
formed by use of the tool used in finishing joints, and 
are of material benefit in preventing the slipping of 
horses. 

At street intersections where the number of pedes- 
trians is large it is desirable that the crossing be carried 
across on the level of the top of the curb without 
leaving a step at the gutter crossing. This may be 
accomplished by bridging over the gutter with a flag- 
stone or iron plate, or by placing the outlets for surface 
drainage a few feet back from the corner and eliminating 
the gutter at the corner. 

Art. 100. Street-railway Track. 

Track for street railways upon paved streets should 
be constructed with a view to offering as little obstruc- 
tion to ordinary street traffic as possible, while per- 
mitting the ready operation of the railway. These 
two points are apt to conflict, as the interest of the 
railway company in the construction of track is rarely 
identical with that of the public use of the street. 

An3^ street-car track is objectionable on a paved 
street, both on account of the increased wear caused 
to the pavement, and because it forms an obstruction 



386 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

to the ordinary traffic of the street. It is, however, 
a necessary evil, being required for the convenience 
of the public, and its detrimental effects may often be 
greatly lessened by proper attention to the methods 
of construction employed. On smooth pavements 
properly constructed track should offer no obstruction 
to vehicles crossing it, and afford no channels in which 
the wheels of vehicles may run and which prevent 
wheels readily leaving the track. 

This requires that the surface of the pavement be 
flush with the top of the rail, and that it be laid in 
close contact with the rail. 

It is also important that the method of construction 
used in both track and pavement be firm and substan- 
tial to prevent unevenness due to the yielding of the 
track or settlement of the pavement. 

Construction of Track. Methods of construction 
used for street railway tracks are extremely various 
and opinions differ widel3r concerning them. When 
the traffic of the railway and street is light it is gen- 
erally conceded that the most economical method is 
that of placing the rails directly upon wooden cross- 
ties, as in the construction of steam roads. Where, 
however, the traffic is heavy the difficulty and ex- 
pense of making repairs becomes great, and the rail- 
way companies commonly recognize the advantage of 
solid and permanent construction. Several methods 
have therefore been devised for securing firm support 
to the rails. 

Fig. 41 shows the ordinary method of construction 
where a concrete base is employed for the pavement 
and the tie is embedded in the concrete. In this con- 
struction the track is surfaced up by ballasting in the 
usual manner under the ties with gravel or broken 



CITY STREETS. 



387 



stone, after which the concrete base is filled in between 
and perhaps over the ties. The depth of rail is some- 
times made the same as the thickness of the upper 
layers of pavement, thus bringing the top of the tie 




Fig. 41. 

even with the surface of the concrete. Thus a six- 
inch rail maj^ be used with a brick pavement having 
a two-inch sand cushion as shown in Fig. 42. If the 
depth of rail be less than this, stringers, as in Fig. 43, 




Fig. 42. 

or chairs, as in Fig. 44, are necessary to raise the rails 
to the level of the paving surface. WTien stringers 
are employed they are usually connected by cast-iron 
braces to the cross-ties, and are also bedded and held 
in place by the concrete base of the pavement. The 
ties in such case are usualh^ below the concrete. To 
secure greater stabilit3^ when the rails are supported 



388 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

by cross-ties a bed of concrete is sometimes placed 
under each tie and the track is tamped in concrete. 
This is shown in Figs. 42 and 44. In such construc- 
tion it is usual to make a trench under the tie, fill this 




Fig. 43- 

with concrete and tamp the tie with concrete, after- 
ward placing the concrete base for the pavement be- 
tween the ties. 

On important lines in streets of heavy traffic re- 
pairs to track are often both difficult and expensive. 




Fig. 44. 

and very rigid and substantial construction is essential 
to an economical operation of the railway. To secure 
such construction the wooden ties are sometimes dis- 
pensed with and the rails placed directly upon the 
concrete. Several methods of construction of this 
character have been employed. Fig. 45 shows the 
simplest form, where the rails are placed directly upon 



CITY STREETS. 



389 



the concrete base of the pavement and spaced by iron 
tie-rods at intervals of six or eight feet. The con- 
crete in this construction is usually made extra heavy 
in order to adequately support the rails and maintain 
them at the level of the pavement. 

A more economical method of securing permanent 



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construction is by the use of a concrete beam or 
stringer under the rail. This method of construction 
is illustrated in Fig. 46. The concrete stringers are 
usually made of Portland-cement concrete from 9 to 




Fig. 46. 



12 inches in depth and 12 to 18 inches in width The 
rails are commonly held in place by iron ties, to which 
the base of the rail is bolted, as shown in figure. Fre- 
quently light angle-bars are used for ties, but various 
other sections have also been employed for which 
special advantages are claimed. Spacing rods between 
the webs of the rails may also be emplo3^ed as shown 
in Fig. 45, but these rods are objectionable in a block 
pavement on account of the difficulty of paving be- 



390 A TEXT-BOOK ON ROADS AND PAVEMENTS. j 

tween them. The ties are usually spaced about ten 
feet apart. 

In some instances^ where concrete beams are used 
under the rails, ties are omitted altogether and the 
base of the rail is spiked directly to the concrete or 
bolted through the concrete beam to plates below. 
This latter method has been carried out at Rochester, 
N. Y., with entire success, no difficulty being experi- 
enced in holding the rails in position. In construct- 
ing such track the rails are usually laid on temporary 
wooden ties spaced ten or twelve feet apart and 
brought to line and grade, after which the concrete 
beams are placed and the wooden ties removed. 

Form for Rails. The rails in common use for 
street-railway track are divided into two general 
classes: tee rails, as commonly used on steam roads, 
and girder rails, in which the head is so formed as to 
afford a channel for the flanges of the wheels and ad- 
mit of the pavement being laid close against the rail 
on both sides. 

Tee rails differ considerably in their details and 
weights and are often modified for street service by 
making them of greater depth than is usual for steam- 
road service. These rails are shown in Figs. 42 and 
43. The upper surface varies from 2 to 3 inches in 
width and is usually made convex, the section being 
frequently circular, of radius 8 to 20 inches. As used 
for street railways, these rails vary from 4 to 7 inches 
in height. In using the smaller depths it is necessary, 
except for very thin paving surfaces, that the rails be 
supported on chairs or stringers to give room for pav- 
ing over the cross-ties, and deeper sections are there- 
fore more commonly used. The six-inch depth is fre- 
quently employed and is sufficient with an asphalt or 



CITY STREETS. 391 

brick pavement, the ties, if used, being embedded in 
the concrete foundation. 

The disadvantage of the tee rail consists in the fact 
that the pavement cannot come against the rail on the 
inside of the track, as it has no groove for the wheel- 
flange. The pavement must therefore either be low- 
ered under the rail-flange or a groove be left between 
the head of the rail and the pavement. The first 
method, shown in Fig. 42, is ordinarily the best con- 
struction, as the pavement is set firmly against the rails 
and there are no exposed edges to cause rapid wear, but 
it is objectionable on account of the impact of wheels 
crossing in dropping from the rail, and because it tends 
to hold the wheels of vehicles in the track. The 
second method is accomplished by using a thin block 
or a filling of concrete under the head of the rail and 
paving against this filling, as is usual when stone-block 
pavement is employed between the rails, or when a 
toothing of stone blocks or bricks is emplo3"ed with 
an asphalt pavement. For brick pavements special 
bricks are sometimes molded to fit against the rails, 
leaving a groove for the wheel-flanges, as shown in 
Fig. 43. Difficulty has sometimes been met in the 
use of these bricks on account of their tendency to tilt 
when the car-wheel flanges press down any dirt or 
small gravel which may fall into the groove, unless 
they are ver}^ firmly bedded next the rail. Tee-rail 
construction is verj^ commonly preferred bj^ railway 
companies, as giving a better road for operation. It 
affords cheap construction, has little tendency to be- 
come clogged with dirt, and will usualh^ be avoided 
b^^ the ordinar3^ traffic of the street, not affording good 
channels for the wheels of vehicles. 

Girder rails are divided as to form of head into 



392 A TEXT-BOOK ON ROADS AND PAVEMENTS. 



centerrbearing, side-bearing, and grooved. They vary, 
as commonly used, from 6 to 9 inches in height, and 
each type is subject to several variations in form. 

The center-hearing rail is shown in Fig. 47. It is 
the most objectionable of any of the forms in use, 
there being two channels, one on each side of the 
head, thus offering a double obstruction 
to traffic and causing greatly increased 
wear to the pavement. It is of advan- 
tage to the traffic of the railway because 
it does not retain dirt, and where streets 
are not kept in good condition cleanses 
itself, which is particularly important 
Fig. 47. ^^ electric roads in which the rail is used 

as a current conductor. Its objectionable features, 
however, prevent its use in most places. 

Side-hearing rails are shown in Figs. 44, 48, and 49. 
They are probably more commonly used than any 





Fig. 48. 

other type of girder rail. The tram is from 2 to 3 
inches wide and offers a smooth track for the wheels 
of vehicles, but it is difficult for a wheel to leave it 
and is extremely hard on the street traffic. 

Pavements may be laid against the side-bearing rail 
as shown in Fig. 44, in which the surface of the pave; 
ment is at the same level inside as outside the track; 



CITY STREETS. 393 

or, as shown in Fig. 48, in which the pavement inside 
the track is brought even with the top of the tram of 
the rail. The first method leaves an exposed edge 
of the paving surface, which is commonly subject to 
rapid wear, while the width of tram is sufficient to per- 
mit the wheels of vehicles to run in the grooves and to 
leave the track with difficulty. 

Grooved rails are shown in Figs. 41, 45, 46, 50, and 
51. There are many variations in the form of groove 
and lip designed to meet varying conditions of use. 
The full-groove rail, shown in Fig. 41, has a groove in 
the head usually from an inch to an inch and a quarter 
in width; and when the pavement is made flush with 
the top of the rail it presents no obstruction to traffic 
of the street, and as the groove is too narrow to admit 
the wheels of vehicles it forms the most desirable track 
for use with smooth pavements. It can onK^ however, 
be used where pavements are kept clean and in good 
condition, as the groove is otherwise easil}^ clogged with 
dirt, rendering the operation of the railway difficult 
and expensive. This disadvantage is greater in cold 
climates wdiere snow and ice are common during 
winter. 

For the purpose of lessening the clogging of the 
groove, the form of the grooved rail is sometimes 
modified by sloping the lip and widening the groove 
at the top, as shown in Fig. 46 or, as shown in Fig. 50, 
by making the lip of a less height than the head of the 
rail, thus allowing the wheel-flange to clear the groove 
of dirt in passing. This latter form, however, has 
the effect of forming a track which retains the wheels of 
vehicles, as will any difference of elevation between the 
head of the rail and the pavement between the rails. 

In Fig. 51 is shown a grooved rail with an exten- 



394 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

sion of the lip intended to form a track for wheels of 
vehicles with a view to reducing the wear of the pave- 
ment which commonly takes place immediately inside 
the rails. When this lip is below the level of the rail- 
head it is subject to the same objection as the side- 
bearing rail of forming a track which it is difficult 
for wheels to leave. 

For paved streets, where the pavement is well kept, 
the grooved rail seems to be superior to any other, 
and is often required by municipal authorities, par- 





M^i^;^^^ ^...x^^MMWM^ 



Fig. 49. 



Fig. 50. 



Fig. 51. 



ticularly in the larger cities. For unimproved streets 
or on macadam or earth roads the tee rail is usually 
considered preferable, and may usually be employed 
with no more injury to the street traffic than an}^ of 
the others, while possessing the advantage of economy 
both in cost and operation to the railway. 

Joints and Fastenings. The solid construction of 
track is a matter of importance upon paved streets, 
because of the difficulty and expense of getting at the 
track to make repairs, as well as because of the dis- 
turbance to traffic when the pavement must be removed 



CITY STREETS, 395 

for this purpose. The rail-joints and tie-connections 
are therefore matters requiring particular attention. 
Where no chairs are used, the use of tie-plates to form 
a bearing for the rail upon the tie, and to hold it 
secureh^- in place, is to be recommended, and will 
greatly aid in forming a rigid track. There are a 
number of forms in use which give good results. They 
should be arranged to clamp the rail firmh^ and present 
a good bearing upon the tie. When chairs are used, 
they, like the tie-plates, should clamp the rail firmly 
and give good bearing surface. They should also be 
well braced for stiffness against lateral bending. 

Joints, in the case of track formed of rails laid directly 
upon the ties, or upon wooden stringers, are usually 
made by placing a plate or channel-bar upon each side 
of the web of the rail-ends to be joined and bolting 
through. The use of slightly curved channel-bars 
fitting against the flanges of the rail, as shown 
in Figs. 49 and 51, seems to give good results, the 
spring in the channels serving to prevent the loosening 
of the bolts. This is the most common method of 
making joints. Fig. 50 shows a pair of ribbed-joint 
plates as used for high rails, the center bearing serving 
to prevent the buckling of the plates or the bending 
of the rail at the ends. 

For track in pavements the rails may be laid to 
close joints, no allowance being necessary for change 
of temperature when the rail is fully bedded in the 
pavement. 

A number of modifications of the above joints have 
been devised, some of them passing under the base of 
the rail and supporting it on the tie. Electrically 
wielded or cast joints are also sometimes employed, 
consisting in welding a bar of steel on each side of the 



396 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

end of the rail, or in casting an iron block about the 
ends to be joined, the casting being joined by means 
of the holes through the web of the rails. 

Where chairs are employed to raise the rails above 
the ties, joints are frequently most • satisfactorily made 
upon long chairs or bridges reaching across the space 
between two ties and forming a firm bearing for the 
ends of the rails. 

In order to facilitate keeping the joints tight and 
enable the bolts at the rail-ends to be screwed up 
without taking up the pavement, joint-boxes are 
sometimes employed. These consist of openings with 
removable covers, giving access to the bolts at the 
ends of the rails „ 

On curves, guard-rails are commonly employed. 
Where tee rails are employed the guard is usually a 
second rail placed on the inside of the main rail, leaving 
only room for the wheel-flanges. In som.e instances, 
however, the guard is formed by bolting a flange to 
the main rail. For girder rails the guard is usually 
formed by the use of a rail in which the groove is 
wider and the lip heavier than common, and sometimes 
the lip extends somewhat above the head of the rail. 
Any difference of elevation of that kind is objection- 
able as producing unevenness in the pavement, but is 
frequently used as essential to the proper operation of 
the cars upon the curves. 

Pavement in Car Tracks. The wear of a pavement 
is usually considerably increased by railway tracks 
upon the street. The extent of this wear depends 
upon the nature of the paving surface as well as upon 
the construction of the track. It is mainly the differ- 
ence in resistance to abrasive wear between the rails 
and the paving surface which causes uneven and more 



CITY STREETS. 397 

rapid wear of the pavement in vicinity of the track 
A broken-stone surface, on account of its rapid wear, 
is particularh" objectionable along a line of track, and 
is very difficult to keep in proper surface. 

In case of narrow streets or rough side-pave- 
ments the use of the track for hauling heavy loads 
causes the cutting of the pavement upon the outside 
of the track, due to the gauge of trucks being greater 
than that of the track. This is especialty the case 
where, owing to the use of side-bearing or center- 
bearing rails, the flange-grooves are wide enough to 
permit the wheels of trucks to enter them. 

Where tracks follow country roads it is usually 
desirable, if possible, to place the track at one side 
and leave the center of the street free for the use of 
the ordinary traffic. When a broken-stone or gravel 
surface is emplo3^ed it is common to laj^ planks on 
each side of the rail and bring the pavement against 
the planks, which materially lessens the obstruction 
offered to travel by the rails, as well as the difficulty 
of keeping the pavement in surface. 

The methods of placing paveinents in 'tracks 
depend upon the shape of the rail-heads and have 
alread3^ been discussed. Under heavj- traffic when 
asphalt street surface is employed it is quite common 
to pave between the rails with stone or brick, and 
often to put a toothing of the same material outside 
the rails adjoining the asphalt. This serves to prevent 
the cutting of the asphalt along the rails. Sometimes 
when stone blocks are used in track the concrete base 
is omitted and the blocks are set on a gravel or broken- 
stone base. When such construction is employed the 
track should be very carefully ballasted and brought 
to an even bearing. There is always a tendency for 



398 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

the track to work loose from the pavement and get 
out of surface, and under heavy traffic very firm, con- 
struction is necessary to counteract this tendency. 

Art. ioi. Trees for Streets. 

It is ahvays desirable, wherever possible, to have 
streets, at least those devoted to residential purposes, 
lined with rows of trees upon each side, both for the 
purpose of giving shade and to add to the beauty of 
appearance of the street. 

The most satisf actor3^ way of arranging trees is usually 
to have a tree space between the sidewalk and the curb 
in which the trees are planted in a straight line along 
the street. Sometimes in ver}^ wide streets a tree 
space or parking is arranged in the middle of the street 
with a driveway on each side. Trees should be spaced 
in the rows at such distances as will permit each tree 
when fully grown to spread to its full natural dimen- 
sions, which usuall3^ requires, for trees ordinarily em- 
ployed, from 25 to 40 feet. 

The selection of the variety of trees to be used for 
this purpose must of course depend upon climatic 
and local conditions. Those which rapidly attain their 
full size are usuall3^ to be preferred. The3^ should 
have a graceful form and make a good shade, but the 
foliage should not be too dense. Evergreens are not 
generally desirable for this purpose. Where there is 
plent3^ of room for their development the large-grow- 
ing varieties with light foliage are handsome and desir- 
able. The size, however, must be suited to the space, 
and upon narrow streets, or where the trees are to be 
close to the buildings, they must be of small growth. 
The ease with which the tree may be grown and its 



CITY STREETS. 399 

liabilit3' to disease or to be affected b}^ the contamina- 
tions of a cit3" atmosphere must be considered, as the 
conditions under which street trees must be grown are 
not usualh^ favorable to their best development. 

It is desirable, especially in cities of considerable 
size, that the planting and care of trees be under con- 
trol of the municipal authorities. Trees ma}^ then be 
set with a view to the best general effect upon the 
street as a whole; the selection and planting of the 
trees maj^ be properl3^ done, and the trees after plant- 
ing ma\^ be S3^stematically cared for. 

Art. 102. Selection of Pavements. 

The selection of the best pavement for use in any 
given instance involves a study of the characteristics 
of each material as to its fitness for the particular 
service required, its suitability^ for meeting the local 
conditions under which it is to be used, and its probable 
cost. Local conditions must be taken into account, 
and it is not possible to laj^ down any fixed rules for 
universal application. The availability of materials 
in the locality, and relative costs which vary -with local 
conditions, are frequently determining factors in the 
choice of pavements. 

A good pavement should present a smooth, hard, and 
impervious surface, which may be easily cleaned and 
offers small resistance to traction. The comfort, con- 
venience, and health of people using the street and of 
residents of the locality ma^^ be largel}^ affected by the 
character of a street, and should be the first considera- 
tion in deciding upon an improvement. 



400 A TEXT-BOOK ON ROADS AND PAVEMENTS. 
HEALTHFULNESSo 

The effect of a pavement upon the health of the 
residents of its locality will be affected by the tendency 
of the materials composing it to decay, by its permea- 
bility, and by its degree of freedom from noise and 
dust. 

The permeability of a road surface is important on 
account of the tendency of surface water and refuse 
matter to penetrate and saturate it, and thus cause it 
to become dangerous to health. A continuous sheet 
pavement is the most desirable in this particular, and 
a block pavement with open joints the least so. When, 
however, the joints of a block pavement are properly 
cemented, the pavement may be made nearly imper- 
vious, and any of the pavements in common use, when 
well constructed, are practically impervious to water. 

Noiselessness, The noise made by traffic upon a 
pavement is important not only because of its effect 
upon the comfort of the people using it or living 
adjacent to it, but also because to it are frequently 
attributed many nervous disorders to which people 
in some cities are subject. 

Stone-block pavements are the most objectionable 
in this particular, causing a continual roar, due both 
to the rumbling of wheels over them and the blows of 
the horses' feet upon them. Upon asphalt the noise is 
onlj^ that due to the horses' feet, giving a sharp, 
clicking sound. Upon wood the horses produce no 
appreciable sound; but wheels give a dull rumble, 
generally considered the least objectionable of an^^ of 
the noises made by the more common pavements. 
The noise of wood pavements is diminished by mak- 
ing the joints between blocks small, and a well con- 



CITY STREETS. 401 

structed wood-block pavement is usually the least 
nois3^ of the pavements in common use. The noise 
made bj^ traffic upon a brick pavement varies with 
the method of construction. The clicking sound 
made by horses is less than on asphalt, but the rumble 
of vehicles is greater, the rumble being usuall^^ more 
objectionable with h3^draulic cement than with bitu- 
minous or sand-filled joints, although when proper 
expansion joints are used with cement joints the noise 
is not ex( essive. 

Broken -stone roads are less noisy than any of the 
harder pavements excepting wood blocks, while earth 
roads are the most desirable on this account when in 
smooth condition. 

Freedom from Dust. The dust arising from a pave- 
ment is objectionable on the score of health as well 
as of comfort. For the most part the dust found upon 
cit3^ pavements is produced from dirt carried there 
from the outside. To eliminate this it is necessarj^ to 
keep the pavement clean, and perhaps to sprinkle it^ 
All pavements produce more or less dust, even when 
kept thoroughly cleaned. Stone, brick, and asphalt 
surfaces all give off a small amount of very fine dust, 
which- rises in the wind unless the surface is kept 
sprinkled. Wood-block surfaces are less objectionable 
on this account. Broken-stone roads wear rapidty 
and make dust freely in drj^ weather, unless kept 
sprinkled or treated with oil or tar (see Art. 41). 

SAFETY. 

The safety of a road surface depends upon the foot- 
hold afforded by it to horses under normal conditions, 
and also upon the degree of slipperiness that it may 



402 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

take in wet weather, or under the influence of ice and 
snow in winter. 

A dry earth road in good condition gives the best 
and surest foothold, with broken-stone and gravel 
roads nearly as good. 

The relative safety of the various pavements used in 
city streets is a matter upon which there is consider- 
able difference of opinion amongst authorities. Local 
conditions affect the pavement in this regard to an im- 
portant degree. The dampness of the climate, the 
shade from buildings, the cleanliness of the streets, 
and the prevalence of snow and ice in winter are all 
important. 

Statistics upon the question of relative safety of 
wood, asphalt, and granite have been collected by 
Captain Greene in this country and by Colonel 
Haywood in London, the attempt being made to deter- 
mine the number of miles traveled by horses upon each 
kind of pavement to each accident due to slipperiness. 

The results of Colonel Haywood seem to show that 
of the three, wood is the safest and granite the most 
dangerous, while the results of Captain Greene show 
asphalt to be the best and wood the worst in this 
particular. 

Colonel Haywood's observations were all taken on 
London streets, and are as follows: 





Miles Traveled to Each Fall on — 




Granite. 


Asphalt. 


Wood. 


In dry weather 

In damp weather 


78 
168 

432 
132 


223 

125 
192 
191 


646 
193 

537 
330 


In wet weather 


All observations ..... 





CITY STREETS. 403 

Tlie observations were made when dry weather 
prevailed, and therefore are somewhat unfavorable to 
granite, which is safest when wet. 

Captain Greene's observations were made in several 
American cities, and showed the distance traveled to 
each fall to be, on granite 413 miles, on asphalt 583 
miles, and on wood 272 miles. The observations on 
wood in this series were too few to give a reliable in- 
dication, and it is to be observed with regard to all of 
them that slipperiness is largely affected by the con- 
dition in which the surface is maintained, and it is 
therefore difficult to draw any general conclusions 
which would fit all cases. 

AM hard pavements are slippery when muddy and 
wet, and cleanliness is the necessary condition of 
safety. 

Wood and asphalt, if clean, are least slippery when 
dry and most so when simply damp. Granite, after 
the surface becomes worn and polished, is most slip- 
pery when dry and least so when wxt. 

Under a light fall of snow both w^ood and asphalt 
become very slippery, and in freezing weather wood 
sometimes becomes slippery through the freezing of 
the moisture retained by it. 

No statistics are available as to the safety of brick 
pavements, but it is thought a desirable material in 
this respect. 

It may also be remarked that the danger of a horse 
falling upon any pavement depends very largely upon 
the training of the animal and whether he be accus- 
tomed to the particular surface in question. 



404 A TEXT-BOOK ON ROADS AND PAVEMENTS. 
DURABILITY OF VARIOUS SURFACES. 

The durability of a road or pavement is dependent 
upon so many circumstances connected with local con- 
ditions, the nature of the traffic, methods of con- 
struction, and efficiency of maintenance, that an3^ 
comparison of the various kinds of pavement in this 
respect is difficult and likely to be misleading. 

The qualities which especially affect the durability 
of the road may be partially enumerated as follows: 

(1) The hardness and toughness of the material com- 
posing the surface, upon which depends the resistance 
of the surface to the abrading action of the wheels and 
horses' feet passing over it. 

(2) The firmness of the foundation, which serves to 
distribute the loads over the road-bed and keep the 
surface uniform. 

(3) The drainage of the road-bed, which can only 
properly sustain the loads which come upon it when it 
is dry. 

(4) The permeability of the surface, which should 
form a water-tight covering to serve the purpose of 
keeping the foundation and road-bed in a dry con- 
dition. 

(5) The resistance of the materials of the pavement 
to the disintegrating influences of the atmosphere and 
to the action of the weather. 

The relative importance of these various factors, in 
any particular case, depends largely upon the nature 
and extent of the traffic which is to pass over the 
pavement. 

The amount of traffic to which a street is subjected 
is usually estimated in terms of tons per foot of width 
of street, by observing the number of teams passing a 



CITY STREETS. 405 

given point during certain times, classifying them, and 
assigning an average value of load to each class. The 
wear of the surface will naturalh- be somewhat propor- 
tional to the amount of traffic. The life of a pave- 
ment is, however, affected b}^ other conditions, and 
hence cannot alwa3'S be inferred from the amount of 
traffic. 

Traffic ma}^ also be classified according to its nature 
as heav3^ or light, depending upon the weight of indi- 
vidual loads which are carried. It is the heavj^ loads 
borne upon narrow wheel-tires that do the greatest 
damage to a pavement, and hence the nature rather 
than the amount of traffic determines the character of 
pavement necessary. 

Granite blocks, where a firm unjnelding foundation 
is emplo3^ed, give the hardest and most durable surface 
of any of the common pavements. This is epecially 
the case under very heav}^ loads. 

The durabilit}^ of wood-block pavements under wear 
varies widety for the different t3^pes of construction. 
The better grades of treated wood-block pavement seem 
to have given results, in some instances, second only 
to granite blocks, and the}^ are being used under some 
of the heaviest traffic in the larger cities. The older 
and cheaper types of wood pavement are inferior in 
wearing qualities to brick or asphalt. 

Asphalt and brick pavements when well constructed 
are satisfactory under any but the heaviest traffic. 
The relative durability under w^ear of brick and asphalt 
is a matter of doubt, both materials being subject to 
considerable variations in quality, and showing varying 
results in different localities, due both to differences in 
the quality of the material and in the methods of 
construction. Bitulithic may be classed with asphalt 



4o6 A TEXT-BOOK ON ROADS AND PAVEMENTS 

as to durability, although it seems in some instances 
to have shown greater resistance to wear than ordinary 
asphalt. 

Broken stone wears rapidly under moderately heavy 
traffic, and should be employed onl}^ on suburban 
streets or countr3^ roads used mainly for light driving 
or a small amount of traffic. 

Art. 103. Sources of Revenue for Street 
Improvement. 

Funds required for the improvement of streets in 
cities are commonly derived either from general taxes, 
from assessment upon property in vicinity of the 
improvement, or from a combination of the two. In 
some instances also special taxes are levied upon 
vehicles, or upon business interests which make large 
use of the streets, for the benefit of the paving 
funds. 

Mr. J. L. Van Ornum has brought together* data 
showing the practice in fift}^ American cities. In a 
few of these the whole charge is placed upon the general 
taxes. In a few, the whole charge is laid upon adjoin- 
ing property, while in a larger number the cost is dis- 
tributed between the two in varying proportions. 
Some cities pay for the intersections of streets from 
general taxes, laying the whole cost in the blocks upon 
property fronting the street. Some also pay a share 
(about 20 per cent to 40 per cent) of the cost between 
intersections from general taxes; while in other instances 
the city pays a fixed percentage of the whole cost, the 
remainder being assessed upon the property benefited. 
Some cities pay for the grading of the street from general 

* Transactions American Society of Civil Engineers, Vol. XXXVIII. 



CITY STREETS. 407 

funds, while others inckide the grading in the cost of 
paving and assess it upon the property. 

There has been considerable discussion on the part of 
municipal officers concerning the proper method of 
apportionment, and many different opinions have been 
advanced as to what should be required in fairness to 
all of the interests involved. Some contend that the 
streets are for general public use, and that the people 
of the city as a whole should pay for their improvement, 
thus ignoring the advantage that the improvement of 
streets may be to the owners of abutting propert}^ in 
the increase of values. Others insist that it is fair to 
tax the whole of the improvement upon abutting 
property, claiming that the main benefit is to that 
property, and that when improvement becomes general 
each will have paid his proper proportion of cost. 
Between these two extremes are the larger number who 
recognize the interest of both parties and advocate the 
division of cost between the citj^ as a whole and the 
abutting property in varying proportions. Opinions 
differ as to what these proportions should be, and it is 
evident that the public interest in some streets of a 
city is much greater than in others. Some streets are 
main arteries for travel, others but ver^^ little used, and 
the relative values to the general public and to the 
property owner are very different in the tw^o cases. 

It would be quite impossible to devise any general 
method of apportioning the costs of street work among 
the various interests involved in such a way as to tax 
each in proportion to the benefit derived from the 
improvement. It is generally recognized that the city 
as a whole is interested in the improvement of its streets 
and also that property in the immediate vicinity of an 
improvement is directly benefited thereby. Therefore 



4o8 A TEXT-BOOK ON ROADS AND PAVEMENTS. 

it may be considered fair and reasonable to tax either 
or both for the improvement. A division of cost would 
without doubt be the more equitable method, but the 
feasibility of securing necessary funds for proper 
improvement by one method or the other must be the 
determining factor in selecting the method. In some 
instances where the funds derived from general taxation 
are closely limited, the assessment of the whole cost of 
street improvements upon abutting property makes 
possible an extent of improvement which would other- 
wise be out of the question, with manifest advantage 
to the property taxed. 

In assessing the cost of street improvement upon 
property in the locality of the work, the usual method 
is to apportion the cost upon abutting property in 
proportion to the frontage upon the street improved. 
Frequently the apportionment is made in proportion 
to area on each side of the street within a certain dis- 
tance of it (sometimes half a block) . In a few instances 
a combination of the two methods is used, by which a 
portion of the cost is assessed upon abutting property 
in proportion to frontage and the remainder upon area 
within certain distance. The frontage assessment 
seems reasonably fair and is most commonly employed 



SEP 28 1912 



